The ability to change a username is nice, but there are situations, like in our planned community (a specific group of content professionals), where we do not want brand names and aliases being used in the community. This is a problem we already have in Google+, and we solved it by requiring requests to join. Any account with a brand name or "Pong Lenus" alias (for example) doesn't get in. We would rather not have to manually manage accounts or go on a "suspend account" rampage just to police the jokers and marketers.
So, long story short, it would be nice if you added a preference in the Permissions panel (maybe under a new "Accounts" section) to allow admins of a given site to choose whether users can edit their usernames or not.