Originality, humor and/or silliness is what makes names good in my eyes. No offense to the people who have usernames like the following, but i personally dislike names with excessive use of X's, as well as the word "dragon". I mean, seriously, X's and dragons in your username doesn't make you cool. Your username doesn't make you cool either, for that matter. You are the only one who can make yourself cool.
But anyways, if a username makes me laugh, then i certainly think it's a good name. Also, if someone nails that cool-name-bro factor without being generic, then i think it's good as well.