The worse:
Story hook isn't as strong
Larian's changes to the rules mess with combat balance
Many of the sidequests here seem pointless. You try to find a way to get the tadpole out only to figure out that nobody can actually do it.
as many people here said, BG3 having a day/night cycle could be awesome, especially since light conditions actually affect hit chance in this game.
controlling the party isn't as intuitive. Clicking on a party member speaks to them instead of selecting them. You also can't drag select, instead you have to use the chain party system for grouping.

The better:
you can actually side with the bad guys.
Combat arenas are more interesting than just a flat plane
lots of hidden nooks and crannies and secret passages.
better verticality
better stealth
5E rules allow for more interesting use of positioning.
5E rules prevent buff stacking before every hard fight.