The worst UI offense to me is the new playlist view that came with their new desktop UI last fall. They went from having seperate columns for Song Title and Artist Name and bundled them into two rows within a single column. Not only does this make scanning through a long list of songs/artists much more difficult, it means they've taken away the ability to sort a playlist by Artist which seems like a major downgrade in my opinion as someone who keeps most playlists sorted by Artist most if not all the time.