Mm, I'm dumb so I don't know about the API upgrades/coding-related stuff, but on the surface at least 2.1 brings you: live wallpapers, 3D app drawer, better contact synchronization, ability to add more than one Google account, and possibly better optimization (not sure about this one, but I understand that many older phones running 2.1--sans live wallpapers--are smoother than 1.5/1.6, though those are all custom ROMs).
