Android

The UI for Android is really atrocious. (This refers to the Motorola Droid version. I haven't played much with HTC's custom interface.) You can customize it so that it's somewhat manageable.

The main problem is with the multitasking. It's a great idea in theory. The problem is that most apps stay open by default, which ends up sucking away your battery life. You have to be smart about killing your apps. That's fine for someone anal like me, but it's death to newbies.