A couple things..
-After mango, if I download more than one app in marketplace, I get a message saying that marketplace is unavailable at this time, try again later, error code blah blah blah. A mango bug?
-Live tile in photo hub stopped working
-A song stays forever paused and always drops down whenever using the volume key, regardless of the amount of time passed since listening to the song.
-No new tile themes..I really thought that was a big one Mango was gonna add. It drives me crazy that there are no neutral colors like black, grey or white..I just don't get it
-Can't customize alert tones
-This might be a personal preference but no folders for apps? Wp7 is definitely intuitive but goes out of its way a little too much to be different, for better or for worse. A long alphabetical list of not just apps, but every single thing on the phone is one giant mess to me. Even the most basic of flip-phones has a "tools" menu or things that are categorized.
-Very, Very limited customization
-No separate keyboard options. You either have to listen to loud popping noises when typing or absolutely nothing at all. Of course you could turn the volume down, but that means the volume for EVERYTHING goes down, incoming calls, music, etc..
-Can't set the lock screen time out for the wallpaper screen. You only get to look at your picture or wallpaper (the only personable customization on this phone) for about 5 seconds until the screen dims and shuts off.
-Not enough settings in "settings".