It just doesn't make sense to me.
It's fine to have characters that are above everybody else. I mean, there always have been some heroes in pretty much any story - hell, even in the bible we have a man who can split seas in two and a man who can live outside his body!
So when I see people being angry at Illidan being such an important character, I don't get it. His story make sense. It's not as if he was born overpowered either. Honestly, if you've followed his story, you'd know that Illidan is probably one of the characters who suffered the most and I don't speak of the Tyrande part.
We all knew that the story was bigger than the Burning Legion too. I mean, hell, the VOID is the biggest enemy in the universe. That's where old gods are created. And with Azeroth being a she-titan who has not yet awakened, it's more than just the bad guys on Azeroth. We're beyond that at this point.
The burning legion is just the beginning and we will need stronger characters than just us to wipe them. If that mean that we have to empower some heroes, why would that be a travesty?