The default World of Warcraft user interface has undergone some excellent changes over the last few months. After the Shattering and Cataclysm, loads of new UI features were implemented and improved upon. In the future, even more tweaks are being added, with the addition of enhanced dungeon maps and other goodies for lore buffs. I can honestly say that right out of the box, the WoW user interface is better today than ever.
However, there are still improvements that could be made, most of them in areas that just haven't been updated in a good long while. I've got a list of three pieces of the UI that I believe could use a makeover.