It’s the first thing potential dates see, and as this humorous article indicates, it makes a big impression (sometimes unintentionally).
I’ve had ever so many users come to me with usernames that either convey something totally off (wrong gender, for example) or just failed to help them stand out and shine.
Your username, coupled with your profile picture, is the MOST visible thing in search results.
Also avoid awkward SMS-speak like “u” for “you” and “4” for “for.” You’ll alienate more people than you attract, even if that is in fact how you communicate in short form like text messages or casual emails. Don’t you usually wait until a couple texts in to get that lazy with someone new? All this tells me is that you’re a dude (probably) who’s named Jeremy (probably) who may or may not have been born and/or graduated and/or created this profile in 1987. So don’t just go sticking in the number of your birth year or area code or whatever.
Numbers = not remotely compelling, unless you manage to work the Fibonacci sequence in there and you’re an actual mathematician or something.
Could send the wrong message about your orientation, despite you self-identifying as straight on the site.