Some of the UI changes in 4.0 just...suck. Hate:
The guild pane replacing the social pane. I opened the social pane with ease nearly all the time, everyone used it often for friends list, who list, guild, etc. The new guild pane just isn't necessary and its bothersome having to use 'O' key instead, breaking that habit.
The new guild interface in itself is too...big. The names on the guild list are too spaced, the old one was better, it had easier access to guild notes, etc.
The new raid frames. Absolutely terrible. If I wanted Grid, I'd download it. It sucks, its too high-tech, unmoveable, I prefer the party frames and such, even in a raid. Really clunky, and bad.
The new pvp pane, it sucks. It doesn't even show your damn Honorable Kills! HK's have been there since pre-BC, why take them away. Right now the only way to see your own HK's is to go into achievements and statistics, which is sad.
I'm such a complainer.