Bad Attila mechanics:

Missile units, like Archers, Crossbowmen, Javelin throwers, Gunmen etc, were completely unaffected by the entire fatigue system. Now reloading ranged units tire, recovery equilibrium is reached at very tired.

Armour values were too low to make +10% or +20% bonuses from items or upgrades be perceived in gameplay so all values were scaled to a larger amount and balanced around higher values.

Pike units had very extreme values when charged with attack cool downs of functionally 0, that and their rigid ness creates very gamey gameplay, and a lack of variety in combat in chokepoints like gates or against very heavily armoured anti infantry infantry.

Personal preference - Construction changed from arbitrary turns to build, to an atleast balanced money system where 1 PO = 100 gold = 10 resource = 10 food

CAI aggression changed, less ai stances like mountain pass ambush to improve aggression, Diplo weights lowered for small actions like trade and non aggression or military access and higher for war and religion.
Results in crusade like early game where armies will travel to cultural borders and wage war (even bypassing neutral cultures like slav armies walking through christian areas to reach muslim factions - depends on which culture is at war with which from the deals of their corresponding factions)
Mid game where bigger factions just keep attacking in one direction.
And late game where trade and marriage make the biggest empires fight irrespective of culture, using tanky armoured armies in proxy wars in civil warring areas
Ingame example would be spain vs muslims sending non aggression treaties to my player europe faction each turn trying to 1 up each other with money offer, they got trade and vassal army support, the one that I didn't go with became very hostile and travelled far to attack me.

Fatigue Changes:

Combat tires faster.
Idle regens faster.
Walking regens slow.
Running does not drain regens very very slow.
Attack hit chance highest when active drops very low at exhausted
Defence highest at Fresh and drops uniformly but not as low as attack at exhausted.
Armour and speed is the same until exhausted where it drops to 50%.
Pikemen arent as lethal at winded, tired or very tired, and can struggle to impale charging units past the first few ranks at exhausted.

Damage Changes:

Missile damage 10x
Crossbow fire rate 10x
Armour 10x
Shield and armour Missile block chance from 15-85% to 80 to 99%
Result is less kills from front, more from back for shield units.
Late plate armour can block 99% of damage from arrows so bolts or gun are needed for AP damage.
To balance 10x armour weapon damage is 6x and bonus is 30x.
AP damage is unchanged everywhere to preserve max hit to kill amount.

Morale Changes:

Wider morale states like Eager, Steady etc.
Slower morale change and bigger jumps in values
No negative morale debuffs e.g surrounded
This is badly implemented in Attila as quick loss already has bad morale no need to debuff unit twice.
Charging -10 morale, frontal combat +30, flanked combat +20, Both +10, Rear +0.
Less debuff for unit loss until +60% unit loss.
Less debuff for army ratio
More chain route - unit considers more enemies so less rout when army winning, less friendlies so one unit chain routes after another in quick succession but the rout and comeback can halt this for a moment.