I usually like usernames that are 3-4 syllables long. It's the perfect medium between being not too short and not too long, or at least that's how I look at it. I also prefer single word usernames to those that use multiple words/names together. That being said, there was a time when my username was Let's Get Lost Together. I liked the creativity of it, but I just felt like after a time, it was too long. Speaking of creativity, usernames that have that special meaning or significance is something I look for in a name.
Gavin also makes a good note that some letters look nicer together than others. I'll admit, there is a slight aesthetics factor when it comes to choosing a username for myself.
A few examples of my favorite usernames on PC, mines included, would be Ausuadriel, Abnegation, Logiedan, Sylphiel, and Sydian. The latter name being one I had for a short time. Good times, right Syd? :p
A username a certain someone once had and I liked mostly for its appearance was, Abercrombie. Which is sort of ironic, given the name in of itself.