I think there's a lot posts before me that go into detail, but BG3 introduces some minor and not so minor changes. Some would be fine and understandable, but the game makes so many of them that in the end pretty much every action, ability, skill and spell work a bit different from tabletop. Core mechanics are still 5e, but it's all heavily modified. This is more of a "5e inspired" type of system, rather than a shot at actual video game adaptation.
It doesn't make the game bad, I'd prefer if there was less changes, but it's enjoyable in its own way. You should absolutely not go into it with a mindset of "I wanna play a 5e videogame" though, you'll be dissapointed.
EDIT: I'd say my biggest gripe with the game is Larian's obsession with interactive environment and surfaces. Nowhere near as bad as in DOS 2, but still too much for my taste, especially for a DnD game. Imagine playing a tabletop game where during every encounter half of the arena is covered in acid, fire, or burning acid.
I feel bad for you. Gigantic Set Piece battles are a staple of D&D, and "Oops, I forgot fireballs fill a volume not effect a radius" leading to entire rooms being on fire is a classic OD&D through 2nd ed AD&D experience.
I mean, any reasonable battle where casters are blowing The Good Stuff is going to be wrecked in a few minutes, and encounter mechanics have been suggested since Metzer, but go off.
No one wants to fight in a white room in person, and even more so when a computer is the DM & could be handling all of that easily in the background. Getting rid of pg. 82 was one of the greatest sins of 5th.
(Pg. 82 was 'suggested damage per level' from 4th edition that was designed to get players to do more than "I move into range, I attack" because trying to do anything other than cast a spell/make an attack was universally sub-optimal. This little one page chart changed all that. It's the spirit of Liarian trying to put environmental effects back in)
