Although there is only one attack button most weapons still have multiple different movesets. You can change basic weapon combo / moveset by holding controller's left stick forward/backward/sideways (holding WSAD on keyboard does the same thing). Forward usually has more stagger/break potential or more single target dps, sideways - sidesteps for getting behind (daggers) or wider swings (spears), backwards - safer attacks (backflips for swords and spears, throwing for daggers). Each royal arm has its own moveset that sometimes resembles its "base" weapon type moveset but never exactly copies it. Weapons in Comrades (co-op) also have their movesets changed.

Another obscure attack mechanic is charged strikes, most weapons (except royal arms and machinery) have them. To perform a charged strike you need to start attacking then pause for a moment and press attack button again. In most cases charged strikes have highest stagger and break chance and can also be used to stop many enemy attacks with long windup animations. For 2h swords charged strikes do wide area spin attack instead.

Many weapons have a gimmick hinted in their description, don't think its just a flavor text. Typical effects are: elemental attacks (subject to all weaknesses and resistances), more damage when user's HP full/low, special effects on warp strike or combo finisher (final attack after a chain).
Elemental weapons (especially with light element because almost every non-daemon enemy has 50% resistance to light) should be used with caution. Elemental damage is not added to base weapon physical damage, instead elemental weapons fully convert damage to a specific element.
Royal arms are always elementless and don't belong to any weapon type for weakness/resistance calculations, but they always have their own unique gimmicks.

All damage from weapons in inventory is shown as strength+attack. This is actually incorrect calculation. The real formula is
Damage = [Strength + Weapon Attack + (Character Level × 3)] × (Attack Damage Modifier × 2)
For royal arms Weapon Attack = displayed Attack + Noctis' Strength (or Magic for staff and bow). That's right, royal arms benefit from strength twice and thus do a lot more damage than you can expect from their stats.
Attack Damage Modifier depends on weapon type and what kind of attack it is (normal, finisher, parry, etc). Normal attack modifiers are 1.0 for swords, 1.6 for 2h swords, 1.1 for spears and 0.75 for daggers. But don't forget that daggers have a very high attack speed.

You never have to precisely time parries as Noctis, simply holding defend button is enough. In fact you don't even need to wait for on-screen prompt. You can hold defend whenever you want for as long as your want (unlike Gladio and Ignis who do consume MP when you hold defend to block/dodge), all parryable attacks will be parried and all dodgeable/phaseable attacks will be evaded.
There are very few cases when timing defense as Noctis is rewarded. For example ring's holy spell does more damage. Some unlockable skills also make timing slightly more rewarding.

With royal arms defending against gunfire has different effect. Instead of phasing through each attack (which can fail if attacks occur too often) Noctis will simply "absorb" bullets. This doesn't fail regardless of the number of attackers and also has no MP cost.

Royal arms health cost (self-damage) depends on weapon itself (sword of the father has lowest cost), attack used (warp strikes cost a lot more than all other attacks) and number of enemies hit. Basically this means don't warpstrike a group of enemies.

Gladio can parry a lot more attacks than Noctis. In fact he can parry every attack Noctis can phase through. The catch is that you have to press defend button at the right time, otherwise he just blocks attacks.

Prompto and Ignis can't parry but Ignis can counterstrike. Ignis' counterstrikes are executed by attacking after cancelling enemy damage with "impervious". All bros have impervious by default but Noctis needs to unlock this skill in combat ascension grid. Basically you just tap defend after getting hit. Only Ignis has on-screen prompt for this and only he can counterstrike after canceling enemy damage. It works with almost all enemy attacks, not just attacks that are normally parryable. The catch is that you have to survive the damage, you can't do this if enemy attack drops your HP to 0. Counterstrike damage is boosted slightly if you react sooner.

As Ignis you can leave danger state much faster without using items by holding total clarity button.

In combat you get 1 AP for each monster finished with a link strike, a parry (this doesn't include Ignis' counterstrike), a warp strike or any bros special attack when you control them (like total clarity or crackshot).

Armiger Unleashed (unlocked by interacting with Mystic's statue in Keycatrich after collecting all 13 royal arms) tutorial lists special attacks as "X number of attack presses followed by warp button press". While not technically incorrect proper and much more precise timing to press warp button is indicated by Noctis glowing white. You can replace "X number of attack presses" with holding attack until Noctis starts glowing, then after a small delay (for some reason glowing start and button press window isn't perfectly aligned) press warp button. Noctis glows 3 times during his normal ground combo, each time leading to different special attack.

Car coating (the stuff that makes your car collect dirt at a slower rate) also reduces self-inflicted damage. Best coating (from a shop in Altissia) makes it impossible to damage Regalia by crashing into something (unless you crash Type-F in flight, in this case it's instant game over). Monsters can still damage Type-D with their attacks but only while you are driving it. Nothing can damage Regalia when player is outside it, even without any coating upgrades.

Only Hammerhead has a "special" refueling/cleaning cutscene but refueling at any other gas station cleans Regalia too.

Because Type-F doesn't use fuel the only way to clean Type-F is to speak with Cindy and ask for maintenance. Cindy will fix dirty but otherwise undamaged Regalia only if it's Type-F. Other types are meant to be cleaned by refueling. This means there is normally (without damaging on purpose, removing/reinstalling parts, etc) no way to clean other types with installed magitek generator (which provides unlimited fuel), that makes coating much more useful.

Entering and leaving any Type-D race fully repairs Regalia for free.

Type-F can fly much faster at higher altitude. Engine upgrades (super and turbochargers) only affect it's ground car mode speed.

Regalia has 2 "secret" colors, iridescent red-black and blue-black, unlocked by owning a certain amount of other colors.

There are more treasure spots in chapter 15 (or any chapter in new game+), added spots contain some endgame weapons and high tier accessories. You don't have to worry, none of the weapons are unique to those spots, everything can be obtained in different ways (quests and menace dungeons) in chapter 15. Few examples:
Iron Duke (2h sword) - three valleys, near one of possible timed quest locations
Organyx (daggers) - near galdin, on top of rock arch above the road
Hyper Magnum (gun) - near fence surrounding fort vaullerey

For cave-type menace dungeons (chapter 15) there is an easy way to tell which way is correct way without looking at guides. If you can see but can't target enemies from another side of the door this is the correct way, and if you can target enemies this way leads to a dead end. Usually it's a good idea to explore dead ends first for more loot.

You can collect items or activate nearby objects by holding interact button. Hold interact and move, Noctis will perform an action as soon as he gets in range. This can save you from a lot of unwanted jumping.

When you sprint and stamina bar is almost depleted (about 1 pixel remains) quickly press sprint button again for instant stamina refill. Otherwise (or if you miss the timing) stamina bar becomes red until it's refilled to at least 50%. Red bar prevents warping and doing this refill trick, it needs to be green for both actions.

This one is shown on a loading screen but still easy to miss. You can refill chocobo's stamina by drifting. Use it by taping "brakes" just before gallop speed boost ends to avoid full stop.

You can avoid fall damage by "dodging" it. Press defend button just before landing. Works with every playable character and with any height.

Resting at camps doesn't fully clean boys' clothes after a long day. You need to rest in town for proper cleaning.

Bestiary lists items you get from enemies as "drops" and "bounties". But it doesn't mention that "drops" is what you get from breaking appendages and "bounties" is what you get from simply defeating enemies.

Additional chocobo colors can be unlocked by doing Wyz's quests and also collecting pears in chocobo races. You can find pears locations here https://www.youtube.com/watch?v=XQ2UhM2FnO8