Illidan Stormrage is the next step down on the list of not-so-bad bad guys -- as we have seen, most of the fighting he's done since Tyrande Whisperwind released him from prison has been against the Burning Legion, unless of course that time he was working for the Legion -- against Arthas! I admit that I haven't yet visited the Black Temple and done the related quests, but from what I read, it seems that Illidan isn't so much evil as he is just ... paranoid. In fact, there is a major plot hole in this question of why the Alliance and the Horde are at war with him. Why did he have blood elves attack Shattrath, when in fact he and the Naaru have a common enemy in the Burning Legion? It would have made a lot more sense for them to work together; and clearly his mistake caused the "good guys" -- not the Burning Legion -- to be the cause of his doom. If he had not inexplicably decided to wage war on us, it would be very hard to argue Illidan is not beyond redemption.
This sort of conflict between imperfect heroes on the Alliance/Horde side, and tragically messed up villains on the other makes the game all the more interesting. With the introduction of death knights to the game, as well as comments from BlizzCon, I get the feeling that Blizzard is trying to give us the sense that "this could happen to me too. I could become a villain like those I fight against."
In fact, why couldn't I? The next step I would like to see in WoW (which only an MMORPG could take), is for players to somehow be able to take that dark path as well -- to become the very villains and raid bosses that other players must work to defeat. They joked about this idea at BlizzCon, and it's true that there would be some tricky problems to solve. You couldn't just give Frostmourne to Private Ownusohard and make him the new Lich King. It would have to be a temporary conversion to the dark side, perhaps as part of a long but repeatable quest, after which the player could be redeemed and everything would go back to normal.
It would be a complicated thing to work out, so who knows if we'll actually see players becoming villains. Still, it seems to me that Blizzard is setting the stage for greater moral ambiguity in players' choices and activities, both in the enemies we fight and in the quests we undertake. Perhaps in Northrend or sometime thereafter we may find ourselves doing quests and fighting enemies, but suddenly unsure which side is good and which is evil.