Actually, I would say that for the average user, a well done OpenID login is considerably less complex precisely because they don't need to make up another user

Actually, I would say that for the average user, a well done OpenID login is considerably less complex precisely because they don't need to make up another username/password to memorize or otherwise keep track of. You need look no further than any of the stackoverflow related sites to see a very successful large scale site using OpenID. I'd also suggest looking at tripit.com, catch.com, mindmeister.com or springnote.com to see excellent examples of OpenID login done in a way that's very user friendly while also providing the option of a traditional login for people who prefer it.