Besides the cameras which are only good for video chat (I was pretty shocked at how bad taking stills or video were, even with the absolute lowest expectations), I have to say I'm pretty disappointed about the iPod app for music playback.
It's a lot of taps to get between the various windows, and things that should be simple like giving star ratings or adding a song to a playlist are really really cumbersome. As well as some things that are just plain buggy, like scrolling to the bottom of lyrics doesn't stay there, after a couple seconds it pops back to the top. I really hope that app is way better in iOS 5.
Video playback app is pretty clunky as well, no video playlists, hard to find music videos, TV shows sorted oddly.