I don't think we'll argue that it should be simple to login, but I'm not sure how what your suggesting makes it easier. When the we have the same principle as everyone else already. Enter username then password and hit login/enter.
Actually, there is an art to usability. Tumblr has it nailed, but for a forum, not so sure (perhaps too obtrusive?). Either way, I can't criticise a community software package as they have their own thought process.
MyBB is a tool; while it is great on its own, it's up to you to create a truely unique product/service. We think we've got a nice thing going in 2.0 (I like it much more than vBulletin) so I doubt we will it change it.
Well what I mean about Tumblr is that I love that registration is just three fields, and that you can register or login using the same form. That's just brilliant, to me.