A good username is something that is something unique, that we might all get a laugh at, some words you don't hear, you don't usually hear people saying Complexity or Qubic that often, but you might hear Kill or Killer or Ninja or whatnot more often.
Redhawk14 has no general meaning, it's the only thing I could generally think of at the time, and I just put the only 2 numbers I could just push at any random moment, since Redhawk was already taken.
Really a username is what people know you by. To be honest the definition of a good username is something like mine, or Kradel or Shook, pretty much everyone in this thread has a good username, as long as it is nothing like 'xXxResaxXx' or 'xXx178xXx', like Shook had said in his post.