re: #1: the avatar and the user profile page will probably break since we're using the username as part of the URL. That's a bug:
re: #2: there aren't currently, but there will be as part of the fix for the above mentioned bug
re: #3: I'm guessing that commas may very well be a character not allowed in usernames as part of the bug fix for CS-1294.
Perhaps you could specify what's allowed in the registration screen itself. I think most people don't realize you can put spaces and punctuation in there, although also don't think most people want to type in whole complicated phrases for their username, so not sure I want to advertise that.
Also please check for other crazy characters. I just got somebody on our production jive forums system who registered with a username of "Whatever?" (that's a superscript TM character at the end). On forums you can't send him a PM, and a test on clearspace shows system errors when you try to go to his people page.