I'm not really very familiar with Greyhawk (so apologies if I'm way off the mark here!) but I get the impression that it wouldn't really be different enough from Forgotten Realms for it to make any sense releasing it as a separate setting. Something like Dark sun or Eberron seem a lot more distinctive...
The Greyhawk setting doesn't need a lot of rules changes, no... but it does look like it could a bunch of setting-linked backgrounds. It could use a few more of the setting specific subraces.
Really, what it needs is a gazeteer, not so much a rules-book.
Thing is, for Greyhawk, FR, and Mystara, 99% of characters would be in the PHB classes.
The subraces could be tweaked somewhat to the various settings (such as explaining what the hell a Gold Dwarf is in game terms in the FR setting).
And while Mystara is most noted for its immortals... it's also the setting where Orcs, Goblins, hobgoblins, sprites, brownies, sphinx, centaurs, satyrs, skynomes, lycanthropes, and merfolk are all available as PC races. Amongst many others. Also, PC's could gain levels in overland merchant and merchant-prince...