Variety and tactics.
You have so many character options that you can put together odd parties. Try a party of midgets, a stealth party, an offensive cleric, an evil party (that mostly chooses the "jerk" responses in dialogue), a party with three druids, etc, etc. Replay value is not really in the story (as it is in BG), it is in tactics and character roles within each particular party.
There are also so many spells in the game - most people only use 1/4 of them regularly, and it's fun to use the other ones. My all-spellcaster party started as an attempt to learn how to use all the clerical spells, as I had never really experimented much with them before.
I also tend not to play games exhaustively. I'll rotate between two or three (right now it's IWD 1, IWD 2, and Disciples 2), and pick a different one if I feel I'm getting bored. I also have limited playing time, so I might go a month without playing one of the games in my rotation.
For a challenge, take a party through Heart of Fury mode (NOT from scratch; you want a party that's gone through before). There are plenty more, and more powerful, magical items, and you will have a chance to bust out the most powerful spells in the game, all the way through. There are also extra monsters added in, so you won't quite know what to expect in each area. You don't hate goblins until you've gone through the Prologue on HoF.
