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.
