@I R Paladin: I'd take a little exception to your claim that Blizzard was recycling Ragnaros; elementals being immortal outside of their home plane has been canon since Vanilla, and it was never even suggested that we did anything but send Ragnaros home back in MC. Really then, we've had two (three?) recycled villains: Kael'thas and Nefarion/Onyxia. It's hard to complain too much about Kael; turning in the trophy item from him explicitly shows that he's still alive and that he'll be back. And Nefarion and Onyxia (ignoring Ony's anniversary revamp and buff to 80) were both resurrected (literally and figuratively) years and years after their original storylines.

The main problem with Illidan's death in BC is that, as it is, it resolves exactly two storylines: He and Maiev and the Broken. Unfortunately, those are the two least important storylines he's in. At its heart, Illidan's storyline is that of two brothers. Conflict between brothers is literally a story as old as time - just look at Cain and Abel - and there's a reason it's no prevalent: Like all archetypical stories, it resonates deeply with everyone. Malfurion and Illidan, their relationship, their rivalry over Tyrande, their dimetrically opposed paths, these are the most important Illidan stories, and right now they're hanging.