Well, this could be a while. I'm going to explain what I always judge via Xbox 360 when I play online:
1) Clan names: I don't mind them, but they have to be neat. For example, if I were playing Team Flag in Halo 4 with the following: PC Mac, PC Castform, PC Axeliira, PC Sheep, PC Sector. Then that clan is legit with the names. But if they went by: PC M4C, PC xCaStFoRm, PC Axeliira90, PC xoxSheepxox, PC S e c t o r. Then I'd have to say your creativity sucks with the names. That's just my view.
2) I dislike usernames with numbers, PERIOD. Sorry to [supporter]Castform[/supporter], but [supporter]jellicentfan1[/supporter] simply wasn't that good ;) I understand all the names that you like might be taken, but be creative with something else. You might like it better, be well known with that. Another thing, I see more kids with numbers in their names like that, such as BrianIsPro53 or whatever, on Xbox Live. I was formerly Yami Slifer, before I made a change to [supporter]Sector Revenge[/supporter]. When I made the transition, my nephew then started using my name(I'm a role model to my nephews because of my gaming success, apparently). On PS3 however, he had the name Yami Slifer7. See a number?
3) I don't like names that don't have a rhythm to it. For example: Revenge Sector. Does that look right or even feel right when you say/think it? Yeah. I like names are original or are good with the rhythm that can be looked/treated as a first and last name, hence [supporter]Sector Revenge[/supporter]. I prefer a space between them, but because you cannot space on some sites or places, its okay with an _ or nothing in between xD I think its stupid though, but whatever.
4) Like many have said before, the x's, like xxxBlahISuckxxx or XxXBlahISuck or xXxBlahISuckxXx or xoxBlahISuckxox, etc. While it can look cool for some usernames, I think its a pointless thing. I rather it be Blah I Suck or BlahISuck
5) Mispelling words (BlahISuk or BlahISux even, slangs are lame) or words scrambled together, like Esper said previous about hithereimjack, have spaces between every letters in your username (Example: S e c t o r) or names in ALL capitalized, such as ESPER, looks like you're just careless about the name & want to get to the point what you signed up for or in a rush for something.
Yeah, there's my 2 cents. Enjoy.