Names I like are those that are more or less easy to remember, but more importantly, allow you to remember them by name... though in the case of PC, Usernames change so often, its pretty hard to keep track of everyone. XD; So long as the overuse of symbols is as minimal as possible, I don't mind, either. You could say I'm one of those people who see Usernames as a means to recognise someone; nothing more, nothing less. ^^
Though there are a few common things in Usernames that really bug me. The primary one is numbers, which I hate seeing in any kind of Username. I don't know why, but to me, numbers just make a screen name look horrible. Maybe its because of "411 7#15", but thankfully, that doesn't always happen on the Internet. Speaking of, symbols are another thing I don't really like, unless only making up a small, almost unnoticable part of the Username, such as, to make an example of myself, my own Username.
Finally, ridiculously long Usernames and Usernames that are all, or almost completely made up of block capitals, are something that don't annoy me as much as numbers and the overuse of symbols, but it bugs be, nonetheless. It kind of gives me the feeling that the member behind the name is trying to be bigger than everyone else.
These are mostly personal preferences for my own Usernames, but there really are some out there that take the biscuit for ridiculousness.