It is indeed fair to ask how memorable or customised IWD's encounters are. It can feel very much a slog if you need every fight to be a special set piece, if you really find IE combat a drag, etc.
But usually the big comparison point is BG2's mage battle strategy. BG2's enemy line up does feature a lot of special enemies and custom-made abilities and the like, but a lot of what drives it is the ability to plonk a couple of spellcasters in a room and have that really drive the combat situation. In IWD, for example, you can sort of stroll along without any kind of spell deflection/save spells, just because enemy spellcasters are few, their spell selection is limited, and they are pretty easy to pick off or interrupt. Think how easy it is to deal with the spells of the skeletal mage in the initial entry to Kresselack's Tomb, compared to how you've got to build all of your plans around not your spells but the enemy's spells if you happen to run into Garrick's woman or the Red Wizards early on (and you don't just stealth cheese backstab fireball them in 0.1 seconds).
Honestly, I like having this range. IWD could have done really cool things had it gone the BG2 route (and come out after BG2 with more development time, I guess), but there's something nice about IWD's more stripped down approach, and the kind of parties you build in response.
