Fixes:

Not being able to build a Plantation on Floodplains with Cotton.
Not being able to build a Plantation on Floodplains with Sugar.

Fixes:
Game should no longer fail to start without Real Strategy mod.
Environmentalism now works as intended (was Rapid Deployment).
Marshes should no longer receive double the benefit from adjacent pastures.
Embarked units should no longer get land movement penalties, for lacking horses/fuel.
Units should heal properly in City-centers and districts (25 HP/turn).
A lot of other fixes.
A lot of esthetic changes when it comes to text formatting, and adding info missing in the vanilla Civilopedia.

Techs / Civics:
A lot of mostly esthetical changes to the Tech tree in the early Eras (see screenshots).
Major changes to Techs in the Atomic and Information Era: Units and other items have been reorganized in a more chronological fashion, based on the vehicles that the in-game models are based on.
Cattle pastures now give Niter with Metal Casting (was Gunpowder).
Settlers are now unlocked by Code of Laws (in order to make the player aware of bonuses for coastal cities).

Terrain:
Ocean tiles now have a movement cost of 5 (to slow down early exploration).
Coast/Lake terrain now yields 0 Food (was 1).
Defending across a River now gives +10 CS (was +5 in vanilla).

Features:
Marshes can no longer be placed next to Snow or Desert terrain, nor Ice.

Improvements:
Many changes to Fishing Boats, like no longer any minus food from adjacent Fishing Boats (Since food from Coast is now reduced initially).
Many changes to Farms, like for e.g. +1 Production for adjacent Pastures.
Pastures bonuses to adjacent tiles has been changed.
Camps bonuses to adjacent tiles has been changed.
Farming improvements can no longer be built adjacent to Snow/Ice.
Missile silo, Solar Farm, Wind Farm and Offshore Wind Farm now have -1 Appeal (NIMBY)

Districts & Buildings:
Coastal Cities border expansion rate is 10% higher.
City-center and Harbors now give adjacent Coast tiles +1 Food (no stacking).
Many Harbor building effects have been changed.
Granaries now give the city +2% to population growth for every Farm, Fishing Boats, Fishery, Polder and Terrace Farm improvement.
Granaries now give the city +2% to population growth for every Plantation on Bananas, Olives, Spices, Sugar.
Granaries now give the city +5% to population growth if you own the Salt resource (seems not to work when you aquire it through trade or city-state though).
Aerodromes get +1 Gold for every adjacent District, Wonder, Ski- or Seaside Resort.
Neighboorhoods get +1 Gold for every adjacent Ski- or Seaside Resort.

Tribal Villages / Goody Huts:
Reduced weight of Science and Culture huts to 25 (was 100 in vanilla, 50 in 2.7).
Tribal Villages are no longer placed on Snow terrain.

Land Units:
Battering Rams are now unlocked by The Wheel (was Masonry).
Increased gold maintenance of Supply Convoys to 8 (was 3).
Drones now retreat when captured.

Naval & Embarked Units:
All naval and embarked units take 1 HP attrition damage per turn (unless in a district).
All Ships can now escort Great Admirals and embarked units at their speed.
Caravels and later naval units can ignore the extra (4) movement cost of Oceans.
Battleships, Submarines and later units have had their base movement points reduced (It was previously based on the specified max speed of the vessels, instead of the specified speed for optimal range).
Several changes to submarines, increasing their effectiveness.
Several changes to Embarked units movement and maintenance costs.

Governments:
All governments now give trade routes to other Civs +1 Science/Culture for every Tech/Civic that you are behind.

Trade:
Trade routes now gives the receiving Civ approximately 50% or less of the Gold amount that the sender gets.

Traits:
Several changes to Maori traits.
Russia's Grand Embassy trait changed to give +1 Science and Culture per international trade route.
Sumer now has a start bias towards horses.

Policy Cards:
Naval Infrastructure and Economic Union now grant Harbors production adjacency bonus, in addition to the gold bonus.

Roads:
All roads now require Techs.

2.6.1 CHANGES:
======================
Fixes:

Iron Working description said Archers get "+3" Ranged Strength, but it's now corrected to "+2".

======================

Fixes:

Descriptions for Caravel, Frigate and Privateer now show correct movement adjustment for Coast/Lake terrain.
Description for Machu Picchu updated to reflect earlier changes.
Pastures no longer give food to adjcent Districts.
Farm bonus from River now uses the adjacencies system, rather than Modifiers. This could help the A.I. realise the importance of Farms on Rivers.
Overfishing penalty is now accomplished via the adjacencies system, rather than only Modifiers. This could help the A.I. realise the importance of building adjacent Fishing Boats. And it should show a more correct amount of Food in the tooltip for the Builder.
Cartography should now be boosted by a Great Adrmiral (apparently NumItems should've been 0, instead of 1...)
Embarked maintenance increase now applies to all units with gold maintenance (previously civilian units were exempt).

Strategic Resources:

Horses stockpile cap begins at 50 (just like all the other resources).

Buildings:

Palace building now starts with more Science (+2) and Gold (+1) to alleviate the slow Research in the beginning.

Wonders:

Petra wonder now gives +1 food, +2 gold and +2 production (was 2,2,1).

Barbarians:

Reduced number of Barbarian Camps per major Civ to 3 (as in base game) since they were delaying progress in the early game and messing too much with the A.I. Civs.

City-Combat:

Cities are now damaged to 70% when captured (was 90).
Each level of walls adds only 5 defensive CS (was 10).
City attack is now reduced when starting the game in later eras.

Units:

Recon class units now get +10 CS when defending vs. Ranged and City attacks.
Ships now heal 25 HP/turn in friendly territory (was 20).
Many Ancient and Classical Era units have had their starting CS lowered but this can be increased by researching Techs.
Cavalry Units are now reduced to 2 Movement without Horses.
Chariots bonus movement depends on having Horses.
Tanks bonus movement depends on having Oil.
Military Engineer moved to Tech: Engineering.
Military Engineer boosts to all districts and buildings are now 10%.
Military Engineer can't boost wonder production (perhaps I can make another attempt in a later update).
Increased Gold maintenance cost for Modern and Information era naval units, except for Submarines.
Increased CS for Battleship and later era Naval Units.
Increased Range of Submarines back to 2.
Submarines now get +1 Sight Range when they're on Ocean tiles. Uboats get +2.

Promotions:

Recon class - Alpine promotion now gives +3 defense if unit is on a Hill or Snow terrain.
Recon class - Ranger promotion now gives +3 defense if unit is in a Forest or Rainforest.

Great People:

Land units now give +4 Great General points per kill (was 5).
Naval units now give +12 Great Admiral points per kill (was 10).

Religion:

Religious pressure from trade routes reduced to 1 (was 2 in
Changed the enhancer belief Missionary Zeal so that religious units gain +1 movement in addition to being able to ignore terrain costs.

Tribal Villages:

Reduced the weight of villages giving Tech and Civic boosts to 50 (was 100). This should result in fewer free boosts from villages.

Traits:

Norway - Harald Hardrada - Embarked units get a discount of 1 Gold per turn. Berserker unit doesn't have an increased embarked maintenance cost.
Former Marketplace of Ideas and Government based trade bonuses aren't working as intended (there's an upper limit of +1 per tech/civic). Removed until I can code a replacement.
Peter's The Grand Embassy corrected to give 1 Science/Culture per Tech/Civic behind, which is the best possible without further coding.

A.I. favored items lists:

Added Philosophy tech as a favored item for A.I. seeking Science and Military victories (Hopefully this will help the A.I. keep up in the science race).
Changed Chemistry to Medicine for A.I. seeking Science victory (since Research lab is moved there).
Added and changed several techs for A.I. seeking military victory. For e.g. Gunpowder was changed to Metal Casting.

======================
Fixes:

Added text for new Commercial Hub adjacencies.
Updated text for Traders (which still said they had land range 12).
Added text to Fishing Boats that they can be built on Oceans, with Combustion.

======================

======================
Fixes:

City-state Military Center building can no longer be built in 1 turn (caused by me putting too many digits for the 'Cost' attribute).
China's Dynastic Cycle trait now has the correct description

======================
Units:

Military Engineers can now be captured by enemy units, and will become a Builder unit.

======================

Fixes:

Supply Convoys exceeded the intended movement (since it gets its own bonus). It should now function as intended.
Several descriptions updated, for e.g. building descriptions now list the units that they can recruit.

Strategic resources:

Horses extraction rates adjusted. Starts at 1 and gets +1 from Horseback riding, +1 from Stirrups and +1 from Feudalism (total of +4 per turn).
Iron extraction rates adjusted. Starts at 0 and gets +1 from Iron Working, +1 from Engineering, +1 from Industrialization and +1 from Steel (total of +4 per turn).
Niter extraction rates adjuseted. Starts at 1 and gets +1 from Metal Casting, +1 from Industrialization and +1 from Chemistry (total of +4 per turn).
Aluminum extraction rates adjuseted. Starts at 1 and gets +1 from Flight and +1 from Advanced Flight (total of +3 per turn).
Oil extraction rates adjuested. Starts at 1 and gets +2 from Combustion (total of +3 per turn).
Changed placement rules for some resources, for e.g. Coal can now appear on Taiga hills tiles.

Bonus and Luxury resources:

Crabs are no longer superior to Fish, instead they get vanilla yield of +2 gold (was +1 food and +1 gold).
Crabs can now be placed on Reefs.
Turtles are now revealed by Sailing. (was Conservation in agrev)
Turtles yield +1 culture and faith and -1 food. (was +1 science)
Turtles will only be placed adjacent to land (was any coast tile).
Pearls yield +1 culture (instead of faith).
Pearls can now be placed on Reefs.
Pearls will only be placed adjacent to land (was any coast tile).
Wine now yields +2 Culture and -1 Gold (was +1 Food and Gold).
Citrus now yields +1 Food and +2 Gold (was +2 Food).
Cotton now yields +1 Production, +3 Gold and -1 Food (was only +3 gold).
Copper now yields +1 production and 2 gold (was 1 prod, 1 gold).
Having an improved copper resource gives the owning city: +20% production towards Ancient, Renaissance and Industrial Era military units.

Features:

Marshes now start out yielding -1 production as well as the previous -1 food.
Marshes get +2 food and +1 production if it has Rice or an adjacent Pasture. (Simulating Rice paddies and how wetlands have historically been used for grazing and fodder collection).
Reefs now yield 0 gold and faith (was +1 for both in 2.4).
Reefs will now gain +1 gold from building a Harbor district.
Unimproved Forests, Rainforests, Reef and Marsh gain +2 Faith after researching Conservation.
Unimproved Forests, Rainforests, Reef and Marsh gain +2 Faith and +1 Science after researching Environmentalism.

Improvements:

Farms can now be built on Banana resources (if you need food rather than gold).
Farms can now be be built on Marsh with a Rice resource. (was disabled in Agrev)
Farms on Marsh with Rice now get an additional +2 Food after Civil Service is researched.
Pastures now give adjacent unimproved grassland, plains and taiga tiles without resources +1 Food. (To help food gathering without farms and simulate grazing).
Fishing boats can now be built on Ocean tiles after Combustion is researched, but gets -1 Food.
Terrace farms now require 2 adjacent mountain tiles to get +1 food (was +1 food for every adjacent mountain).
Unimproved Coal, Oil and Uranium resources gain +2 Faith after researching Environmentalism.

Routes:

Classical Roads (bridges) now costs 1 build charge.
Railroads now costs 1 build charge but can be built outside territory.
Military Engineers now have 8 build charges, England gets 12 (was 4 in 2.4).
Military Engineers can now boost production for more districts and buildings. See Civliopedia/Civfanatics description.

Districts:

Campus district now gets +1 Science for every resource (since the A.I. seemingly couldn't handle the Modifiers).
Campus district no longer gets science from Resource classes or mountains.
Campus district gets +1 Science for being adjacent to Spaceports.
Commercial hubs get +1 Gold for every adjacent Bonus resource.
Harbors get +1 gold for every adjacent Pasture or Quarry.

Buildings:

Hospital slightly nerfed: now has a fixed science yield that equals half the science yield of Research Lab.
Factories now have a Gold maintenance of 4 (was 2).
Coal Power Plants now give -1 Amenity.
Oil Power Plants now give -1 Amenity.
Nuclear Power Plant now have a Gold Maintenance of 9 (was 3).
Ancient walls now provide Outer Defense HP of +150 (was 200).
Medieval walls now provide Outer Defense HP of +125 (was 100).
Renaissance walls now provide Outer Defense HP of +125 (was 100).

Wonders:

Machu Picchu now gives adjacency bonuses for mountains to Campus.

Techs/Civics:

Celestial Navigation will reduce the movement cost for flat desert tiles to 1.
Combustion will now allow building Fishing Boats on Ocean tiles.
Archery boost now requires 2 camps.
Cartography boost is now to earn a Great Admiral.
Advanced Ballistics boost is now to own 3 Artillery.
Plastics boost is now to build 2 Oil Power Plants.
Drama and Poetry now also requires Mysticism.
Chemistry now also requires Steam Power.
Medicine now also requires Electricity.
Military Tactics now requires Engineering.
Apprenticeship now requires Engineering.

Pantheons:

God of the Sea pantheon now requires a Sea resource in addition to Fishing boats.
Lady of the Reeds and Marshes pantheon now gives +2 production to marshes (was +1).

Units:

Pillaging now costs all your movement points, unless units have the promotions reducing this. (Should stop ships from pillaging all improvements in one turn).
Gold maintenance cost for land units is doubled when they are in enemy territory (except Recon class).
Gold maintenance cost for all units has been increased by 1, except for Settlers, Builders and Traders.
All Support Units, except Drones can be captured!
Great General points from kills increased to 5 (was 2).
Great Admiral points from kills increased to 10 (was 4).
Recon units Alpine promotion now also reduces movement cost of Snow terrain to 1.
Siege units with Expert Crew promotion now get +1 Movement and ability to move & shoot. (was Move & Shoot, which was useless since they start with a movement of 1).
Battleships have +10 CS when defending. (was 60, now 70)
Submarines have +5 CS when defending and range of 1 (was 65 in CS, and range of 2)

Ranged Strikes:

Ranged Strikes were previously all but disabled but have now been changed to the following. Assuming the game begins in the Ancient Era:
Cities with walls will start out with a minimum Ranged Strike Strength of 5 (was 3).
Building an Archer should increase the strength to 10.
Building a Crossbowman should increase the strength to 20.
Completing the first Tech/Civic of the Industrial Era should increase the strength to 30.
Building a Field Cannon should increase the strength to 32.
Completing the first Tech/Civic of the Atomic Era should increase the strength to 42.
Building a Machine Gun should increase the strength to 45.
Note: Starting the game in later eras is not recommended but should result in stronger Ranged Strikes.

Barbarians:

Barbarian Outposts should no longer appear in Snow terrain.

Traits etc:

Marketplace of Ideas is now associated with Governments and give different outputs depending on Tier (was a static +2).
Peter's Grand Embassy trait now gives +6 science/culture for every tech/civic behind, compared to the destination Civ. (was +2).
Maori trait that gives Fishing Boats +1 Food now also requires a Resource.
Maori no longer culture bumb when building Fishing Boats, but instead get +20% border expansion rate in Coastal cities.
Maori now get only +1 embarked movement since naval movement speed starts out high already. (was +2).

Emergencies:

Disaster Aid Emergency now has a lockout time of 30 turns (was 10, just like the duration).
Send Aid projects now have 1/3 the production cost.

======================
Fixes:

Corrected text for Anti-cavalry units to make it clear that their Strength bonus only applies when defending against light and heavy cavalry.
Archers received a general strength increase instead of just ranged strength (from Bronze Working, Iron Working and Construction).

======================
Combat:

Cavalry units no longer get defensive bonuses from Forests and Rainforests against melee, anti-cavalry, recon and siege class attackers.
Swordsman no longer gets its +5 defense bonus against Siege units (They're already at -17 from the base game).
Support bonus is reduced to +2 per unit (same as vanilla) to give more incentives to flank enemies (still at +4 per unit).
All land units get +2 Great General Points for every military unit killed.
All naval units get +2 Great Admiral Points for every military unit killed.
Slingers now gain +2 ranged strength from Iron Working.
Persian Immortals now gain +5 ranged strength from Construction (Composite Bows).

======================
Mods:

JNR's mod: "XP Bonuses and Royal Navy Dockyard Hotfix" has been incorporated, until Firaxis patches the game. Having JNR's mod enabled as well shouldn't cause any problems, if you already have a game going.

======================

Unfortunately, most of these changes only apply if you start a new game, like the changes to