I'm more annoyed by having to pick a (unique) username. My name is too long and too common, all of the nice short versions are always already gone and why the hell am I so often not allowed to separate my first and (abbreviated) last name with a dot? Use my email address as the unique identifier and let me enter my first and last name or a nickname (which doesn't have to be unique), please.
Don't make me think. You should never ever have to show me the "This name is already in use." message. Your design shouldn't even need it. Not everyone has or would like to have an (as unique as possible) nickname on the web they would like to use.
(Unique) usernames are the one vestige of the old web I would like to get rid of post haste. Call me Michael. (I still positively remember signing up to Facebook because I didn't have to pick a username.)