i like usernames that don't have underscores, dashes, xX, or numbers in them. So I guess, my definition of a good username would be something simple, one or two words, but not too long. Also, I'm fine with capital letters, if it's all caps, not a random mixture capital and lower cases. It helps if it's visually appealing, or funny in some way.
I like names that tell a story that I can understand. Though I know what Chibi is due to the internet I tend to prefer names that I would have known the meaning of if I never had the internet.
I hate numbers after people's names. X's and O's and - or _ don't really bug me too much as long as there aren't too many. If there was a xoxo__dragonmonkey_xoxo I'd really be thinking to myself: "really?"
I don't like usernames with anything other than letters, spaces, and dashes in appropriate spots. Adding weird characters just makes it look like you weren't creative enough to find something not taken. I really don't think it's hard to find something! I changed some mulettes recently, and they're Aurienna, Fiorina, and Katralina. Fiorina is a real name even... there's plenty of things still around.
I also don't like people using semi-inappropriate names. I don't mind cursing in speech, but I don't want to be forced to refer to people by something sexual. I had two members in my WoW guild named 'Orcgasms' and 'Bonermania'. Somehow they never were forced to change those, but I was always kinda squicked when I had to say something to them.
I like namey sounding names or words. I'm both of these things. More of my mulettes tend towards the fancy names. I do have two other Storm girls, though. I call the trio the Storm Syndicate, because I am weird.
I like fairly simple usernames. Usually one or two words without an excess of random capitals or punctuation or "x"s. I prefer if they make some sort of sense like a name or a title. Word play is always a fun bonus as is matching some sort of theme for the account. Not so much a fan of random-seeming names like "llama fluff ninja" or "pineapple cheese demon". Usernames that reference awesome shows/books are always a plus.
Anything without the xX, 29343434, or -~ kinds of things. Usernames like Chibi [whatever] or Dark [whatever] are also not very memorable or desirable to me.
That being said, even though I have a short username without any of that, I'm not really into mine. One weird aside is that, even though when I joined there were no other Kaeths on Gaia (the name is made up, obviously), it somehow became a thing afterward and now there are 50+ usernames that contain the "Kaeth". At one point there was this group of friends that had coordinated their usernames around the concept of "Kaeth" ("Kaeth of Death", "Kaeth of Light", "Kaeth of Wind", etc.). They contacted me out of the blue as the "original" Kaeth and we would sometimes chat. It was kinda funny but also confusing because like, the name doesn't mean anything.
for me it kind of depends... most of my actual usernames (on here and else where) are normally based on memorable things that have happened or inside jokes that have been created... but considering i make more rp accounts on gaia than i've ever had of personal accounts and NOT wanting to use X_characternamehere_X or characternamehere1234 and other such things i've come to terms with either using suffixes, character nicknames, or things that describe the character in front of their name like Cute Charactername or Loud Charactername...