Here are a few things that I am aware of:

* Wizards do not get to pick new spells at level up. This means that you are at the mercy of the scrolls that you find.

* Scrolls are scarce. While my understanding is that you can eventually get all the spells, you might not get them as soon as you'd like (so you might get a new level of spells, but no useful spells (or possibly no spells at all) at that level. This also means that there might not be enough scrolls to go around for multiple wizards. For this reason and the reason above, Sorcerers might be better than Wizards in this game.

* Bards get the bard songs they get in Icewind Dale 1, though the song that heals you will only heal you if the game thinks you're in combat. (They still get spells via 3e rules, and therefore aren't scroll dependent the way they are in IWD1, but they don't level up faster, either.)

* XP distribution is based off party average level, rather than the individual character level. If one character is behind on level, that character won't gain extra XP to catch up; rather, the whole party will earn more XP. The game doesn't implement the rule that would prevent gaining multiple levels at once (though when you do, you have to get all of them at once, and I believe they all have to be in the same class), and as a result, this can be used to get far more XP than what the developers intended at any given point in the game. Note that this can easily be gamebreaking, making the game too easy. This also allows you to ruin game balance just by introducing a new level 1 character into a party of experienced adventurers, which is just silly. (I consider this mechanic to be unsound, for reasons that should be obvious.)