People have complained that the hardware is plasticy and cheap, but after using the iphone4 over a 2 month period and seeing the glass screen smash. I prefer the light feel; plus most people covert their phones with a case and they all feel the same after that.
Complaints on the phone:
The browser isn't as good as the iphone; change between pages isn't as nice and the back & forward is a pain. Rendering is also not as good.
The zune navigation system was better than the windows mobile. In zune you could navigate up and down the system by clicking the title of the current screen to go back in Playlists click "Playlist" to go back to music, click "Music" to return to the main screen. This was super convient because navigation could be done with one hand. In the Mobile OS it would more on a backwards only system that requires using the phones "back" button which is at the bottom of the phone. I find it not as easy operate as the zune, but the same as the iphone.