I am not fan of usernames at all. Its unnecessary. The best for me is email and password what I choose, without restriction. On some sites I just use simple password what I remember and its not important to be too secure.
Generated passwords are crapy for me, user still have to change it, there are many unnecessary steps. I think - email and password, works the best. Each additional step makes me nervous.
On sites where "hard" password is required I have usually problem and need to reset password many times ;)

Would be nice if core WordPress implement something really basic, who will want more secure site or something additional can always use some plugin for extend it.