Front touch screen + rear pad + motion sensors could provide some unique gameplay experiences. Having a handheld capable of displaying visuals that rival those offered by current gen consoles doesn't hurt either. Unlike Uncharted, MGS4 seems to be a direct port and though you can tell that it doesn't look quite as good, those are obviously the same assests used in the PS3 version. If they manage to do a direct port of that game with only sacrificing a few visual effects and not some radical changes like reducing the scale of the more complex scenes or even changing some levels entirely, that should tell us what to expect.
Uncharted definitely seems to have visuals on par with the first PS3 game, so I guess we can at least expect visuals similar to those seen in the best games released during the PS3's early years. Now that I think about it, that's pretty much what the original PSP was to the PS2.
I don't see myself buying a new console in at least another 2 years, handheld or otherwise, but both the NGP and the 3DS have compelling arguements.