well, for a start, the graphics obviously will be a lot worse then the ps2. some things which are small on a ps2 screen will either be enlarged on the psp, or not there. some modes or views say an in car view in some games wont be existant, because the psp game's memory is too small.
imo, screw all handhelds apart from iPod touches, because they are good for internet, videos and music. handhelds bore me, so i play xbox360, my laptop or ride meh bike xD