I don't think that the problems of a 4E Greyhawk setting stem from superficialities such as the lack of gnomes or half-orcs. The lack of monks and druids (at least initially) is going to be somewhat more difficult to get around, considering how central both are to some canonical sources, but if they're coming eventually, that can be dealt with.
I am less sanguine about the necessity of introducing wholesale changes in the setting just to accommodate the new rules, however. Choices of races should be setting-specific; Dark Sun has its half-giants, etc. Why do we need to fold, staple, and otherwise mutilate the setting just to accommodate new PC races? The rules should, ideally, be flexible enough to bend to the needs of the setting, rather than forcing the setting to go through dramatic changes to accommodate a change of system.
But what really seals the deal for me is the very notion of the "points of light" concept. It is completely at odds with the underlying design philosophy of the Greyhawk setting, which has, as the movers and shakers of its story arcs, the clash of realms and the behind-the-scenes machinations of secret cabals and powerful individuals (such as the Circle of Eight, the Scarlet Brotherhood, Iuz, etc.).
The Flanaess is not a place where isolated pockets of good struggle heroically against the impending darkness. It is a place where the forces of light and darkness contend openly, in relative parity, where the PCs can make a difference in a close contest.
I just don't understand why 4E seems to force milieu-specific changes on campaign settings. Just look at what we already know is in store for the Forgotten Realms; the whole setting is being turned upside-down to accommodate the 4E changes, and it's not superficial. I have to wonder what will differentiate GH from FR from, say, Mystara, when they all are forced to conform to the philosophical demands of the new system.
Push the timeline ahead a hundred years, collapse most of the kingdoms and remove most of the power centers, and you get a 4E setting. But what do you gain? Just a couple of names? Why bother?
Joe