I'm currently building a social media news aggregation site, and was considering the merits of using a Facebook/Twitter only login/comment services. Do you believe most potential users would be opposed to this? If using these services we would be very clear about privacy issues and options,giving our users the option of restricting their activities to our network, and not sharing the information with facebook/twitter…
Personally, I like StackExchange's model: http://i.imgur.com/7iAgB.png
It gives you some default options, but it still has that "Show more login options …" link for people who don't like them.