I'm seeing problems with a few things here.
1) If you register as "Bob Smith," the system will let you in (temporarily), but when you come back, you cannot login. Why? Because "Bob Smith" becomes "Bob-Smith" and Wordpress isn't nice enough to tell the user that. This has already led to a lot of hairpulling, aggravation and tears.
2) Yahoo, Hotmail and many other free email providers seem to delight in putting Welcome and Confirmation emails into Spam/Bulk. Our new users have no idea about emails going into dark holes, and don't realize they need to Confirm in order to be Active. In part, this is because THEY ARE NOT TOLD THESE THINGS!!!
(Aside: Why is it developers just assume Joe Average User is as web savvy as they are? In fact, as a professional communications guy (Creative Director) who has designed interfaces, I see this time and time again, and it's one of my pet peeves. I consider it the height of arrogance, and very bad interface design. The end result is that people get so aggravated and frustrated they want to scream. "The site doesn't work!" "This is crap!" "Why won't it let me in?!?!?" Many just go away and never come back.)
In order to address the above issues, I've added text all over my site to warn folks about them. But the one place the text really needs to be - the freakin' Registration page - is not something I can modify, apparently!
I realize the above problems are not of WPMUs making (I don't think). They're WP/bbpress/Buddypress issues. But as you are major developers of software within this sphere, I think it's something you should be aware of and think about.