One thing I've always wondered is why there are almost no sound addons around. I mean like 'sound packs' that replace the default WoW sounds. I used to have one, but it got removed.
It'd be great if more people would do this. I'd do it myself if I knew how to create them.
Just imagine tons of different sound packs replacing the UI sounds with something futuristic, or spell sounds with some really cool effects.