Something that users' can't currently do is change their username. It's been asked for on Twitter, and while it's a feature that probably wouldn't be used often, when users do want to change their username it's normally for a very good reason.

I don't know the codebase too well, but from a quick grep last night, I can't see any reason why username needs to be immutable (but I'm sure would be better placed to confirm), so at a minimum we would need to update the user API and edit user controller to allow for a username change, or even better, record the username change so that references to the old username in URLS etc do still work.