The Addons menu is probably the first place a few tweaks need to be made -- as Mend Pet says, it would be nice to be able to load and unload groups of addons with just a click, but let's dream big. How about loading addons from directly within the interface, rather than having to drag them into the Addons folder? I would also like, though it'd probably be tougher to do, to see some kind of graphical interface, kind of a preview of what the addon might look like on the UI. Obviously power addon users customize things so much it'll be tough to tell what it would look like, and some addons have no graphical UI at all. But anything that can let me know from that screen what various addons actually look like or do in the game would be valuable.
Mend Pet has some good ideas for the character selection itself, too -- being able to sort characters the way you want would definitely be nice. Erik in the comments suggests showing other character information, like professions, beside each character's name, and it might be nice to see if the character has any pending mail or guild raid invites right from that screen, too. Of course, that increases the number of queries to the server, but if Blizzard can put it in the mobile app, it can't be too hard to put on this screen, too, you'd think.
Finally, even the character creation screen could use a once-over -- I completely agree with Mend Pet that there should be a way to check names that doesn't include actually creating a character. Other than that, I don't know of anything really pressing -- they mention that it would be nice to view more angles, and I'd suggest that it'd be cool to see various stances or animations, but then again, character selection can't show you the whole game even before you start playing.
Lots of good ideas in there. Obviously updating these screens isn't the most pressing of tasks, but it's true that, especially with a game that's been around as long as this one has, Blizzard shouldn't leave anything off limits when the patch notes come around.