All of the Infinity engine games are pretty similar, but is there anything in Icewind Dale II that wasn't featured in any of the other games?  I'm thinking in terms of character classes and abilities, moreso than enemy types or backgrounds.

I think feats are available for characters.

The biggest difference is probably that Icewind Dale II is based on the 3rd Edition D&D rules, while all other Infinity Engine games are based on 2nd Edition.  Even though quite a bit of content is remains similar, there are still significant and pervasive differences, including character classes, races, core rules, spells, abilities, weapons, and lots more.