Since when did people assume Illidan was swayed by the old gods?
The only thing he was ever swayed by was more power. His gift from Sargeras was one such thing, hell in wc3 he goes after the skull of gul'dan because Arthas told him it had lots of power Illidan is a wow version of a crack addict. He needs this stuff. Same thing for the reason why we have the new world tree. He took some vials and made a new well (which is why he got busted and put in prison in the first place.
The only time aside from the cinematic that we saw much of illidan and heard him speak was in black temple when we're about to fight him. Other than that I think one or two quests where he appears at least in a vision/spectral form. Other than that we just learn of whats hes doing through some quests. We don't get to see much of him at all.
The lich king on the other hand was ever present (almost like a saturdays kids cartoon villain) Garry I feel has just about the right amount of "exposure".
I just think with illidan if they had explained properly why he had turned rather than "just lol insanity" would have been better. If we'd had known that since his defeat he feared the legion would hunt him down so he returns to outland to seal it off and bolster his forces so he can be safe etc and that he wasnt insane just went to extreme lengths to protect him self.