What are the major differences between 2nd AD&D (BG) and 3rd D&D (IWD2)?

Hello folks,

let me state this beforehand, I haven't yet had the chance to play IWD2. However I'm quite familiar with the Baldur's Gate games and the first part of IWD.

Now, I've seen a lot of discussions about IWD2 being based on 3rd edition D&D rules for the first time. Just out of interest, could someone point out the main changes that came with this new introduction.

I've seen a lot of negative feedback on how the ruleset loses its depth and complexity.
I'm planning on playing the game soonish.

Thanks!