From the reviews I've seen, the biggest complaint is the lack of a UMD drive. It's smaller, the controls slide down, and some buttons are in different places. It seems, to me at least, to be intended for watching video as much as for gaming.
While the theory is good, I don't think it'll fly as well as they'd like. They're trying to force a customer base that's had years to build up collections of game discs to switch to digital, without allowing a trade-in program of any sort. It's gonna turn a lot of folks off to not be able to play their old games on their new system. And the $250 price tag will turn off new customers, since they can get the basically-the-same PSP-3000 for $199, or a Wii, or pay another $50 and get a PS3 or X-box 360.
Heck, Sony itself thinks Gos will help sell the older 3000 series. If you're just wanting to replace an old, ailing PSP, I'd recommend just getting a refurbished PSP. It'll be a lot cheaper.