Since this is the attempt to create the next generation of Internet discussions, why does it still hold to the ages old outdated convention of having usernames that have only letters and numbers? Personally, my username is “vilx-” everywhere and I’m annoyed everytime a website forces me to remove the minus sign or (even worse) append something else to it, like “vilx4”, which is plain ugly and pointless.
I understand that there are abusive people who will make usernames that are hard for others to use. Like a username that consists of a single space or something like that. But there are good Unicode libraries today that allow for normalization of strings and identifying of character classes.
I suggest that the authors use such a library and only forbid unprintable characters as well as usernames consisting completely of whitespace (which can be eliminated by whitespace trimming in the first place).
That, or simply do the same thing as with all objectionable content - make a “flag” button for usernames. That might be needed anyway, because even with the strictest rules nothing forbids one to create a username “Fuk0ffB1tch”.