I am not superwell versed in 5e so some of those might be off, but those that come to mind
Reactions - there is no reaction system as such, you don't get to choose to act during enemy turn if circumstances allow. Some stuff, like opportunity attack is a toogle and characters will attack at first opportunity, with first toggled reaction available. We are yet to see what they will do with stuff like paladin's smite, or counterspell, and at the moment to me it feels like reactions are barely in the game. Not something I minded at first, but after paying Solasta, it seems like BG3 is missing a chunky part of action economy.
Free advantage for highground (and till recently attacking from behind the enemy) - that's a powerful boost to chance-to-hit, making buff spells somewhat irrelevant. That's been changed for better in the last patch, as there is no longer free advantage for melee characters. I think rogues can still get it if they attack from behind from stealth.
In stealth there are view cones, that make it rather easy to be invisible as everyone can attack and hide on the same turn. Could be fixed with better AI (at the moment they will stand and do nothing, if there is no visible enemy)
quite a few main actions were made bonus actions - hide, push, drink/use potion, jump (though thank goodness since recent patch disengage is a speperate action that cost's main action)
Scrolls can be cast by anyone (is that Larian or 5e? Not sure).
Surfaces, special arrows and granades, (which create surfaces) explosives (welcome nerf to those in recent patch) moved over from D:OS2 - guaranteed sources of damage, that really mess with concentration mechanic - good luck maintaining concentration when 5 goblins throw guaranteed damage granades in your face.
You can throw stuff - never used it myself but I hear it is pretty broken (don't know if patch 5 fixed anything). Apparently, heavy items, like armors, do quite a bit of damage when thrown.
Treat it as second hand information, but I hear a lot of ACs were changed, and hitpoints increased for the sake of having less misses in combat. Before recent patch I would rarely miss - be it thanks to always having advantage or lowered AC - I don't know. Just recently someone raged on Larian's forum, because they changed Phase Spiders, from stealthy meele enemies, to ranged teleporting away enemies (ranged attacks of course level a puddle of poison). Intellect Devourer's are this game equivalent of "rats", though I am unfamiliar with TT version. I fought level 5 NPC with over 80 hitpoints, which was odd as my high constitution character had about half of that on level 4. I hear 80 is not a norm.
Lots of small things, like rogue's sneak attack being one per turn activable ability, rather then activating automatically with hit. etc.
My impressions is that gameplay revolves a lot around surfaces, and high ground - generally winning or loosing encounter depends on whenever you or the enemy is on highground. Being elevated and pushing down enemies is by far the best strategy one can take. Enemies like to have ridiculus jump and throw distances to counteract being thrown down. Buffing became somewhat more useful to melee characters, but with how many surfaces and guaranteed sources of damage there are, it's difficult to maintain spells for any meaningful amount of time.