Configurability is good. But from a user's standpoint I rather like sites that use email as login key. Otherwise I have to guess which of the usernames I use hasn't been taken yet at that site, and if it's a site I visit infrequently, I have to go through the same guessing process when I come back.
(IMO it's rather silly of people to share a mailbox in the 21st century when you can get free webmail at dozens of sites and even free IMAP accounts at some. Plus all the major ISPs offer multiple boxes per account...)
