It seems that I'm in the minority here among both OT and PT "fans", but I 've always though it was plain and boring natural aging. The Emperor is very old. He's, what, 90 by the time of RotJ ?

I've always thought, way before the PT, that he was very old and just like Yoda his strength in the Force kept him alive. But while Yoda had that aura of a wise, honorable even if fragile looking old creature, the Emperor looked scary but warped, disgusting and hunched.

For me RotS was a huge letdown in this regard, either with a "reveal" or "Force lighthing" cause-effect.