Apologies if this is a duplicate.
It was suggested to me (and so I am passing it on) that we allow login to the projects with an email address as well as a username. So the field would read Username (or email address):
The idea behind this is that for casual editors, remembering one's username is a pain, especially on a big project like Wikimedia where many common usernames may be already taken. The step of trying to recover a lost username and password takes of valuable time and is frustrating and may be a slight barrier to entry for casual editors.