Dear websites that make me register: don’t force me to memorize my username. I don’t care if you send me an email on registration. I hate going through my email to look it up.
I am not going to log into your site often enough to want to memorize the username I chose when I signed up, and I couldn’t necessarily pick my normal one, because some were taken.
So please, let me use my email address in place of my username for signing in. You have it anyway; you required it during registration.