His voice is different.
I always wonder whether he sings differently on purpose or not. For instance, "Lady, You Roof Brings Me Down" has a thicker deep sound to it, while other songs on that great record sound more like his voice we're used to hearing now.
No matter what he does in the future, I just hope he doesn't use so many vocal effects. His voice is great no matter what, and when he uses so many effects and harmonies all the time, it sounds like he's hiding something. In my opinion, Slither would've been better if the chorus wasn't so layered.
I also wonder if "their hits", like Plush or Interstate Love Song would've been as popular if they were recorded later in their career. Or how would Sour Girl or Down sound with his old voice?
Just some thoughts