I agree that, seen from an Apple-booster/altruistic point of view, it would be very nice for G3/slower machines to perform well using X. But, I wanted to point out that even if Apple is shooting for the other 95%, they really don't have to worry about, say, Beige G3 owners. That is, even if the Windowed Masses were to start buying Macs in droves, they'd be buying them now or in the near future, in a world where a 500Mhz G3 is about as slow as you can find (and in a few months, we might not even have G3s outside of the portable arena). So, although X does favor beastly hardware, it should run just fine on the Macs that ship now and in the days soon to come. (And if you think about it, what brand new OS doesn't--does anybody remember how slow system 7 ran on 68020 macs, and 8 on the earliest generation of PowerMacs? Not pretty.)
Anyway, I do feel sorry for owners of newish but slower machines, but X is a heckuva lotta power for a desktop os, and it sure screams on fast hardware. Besides, Apple is making improvements--give them time.
As for me, even 10.0.4 does most things frighteningly fast on my DP533 (though even I can empathize with window resizing), so I'm a happily overequipped camper. I have run it on a G3 266, though, so I feel your pain.