How cool would it be if a user could:
1. login to a WP site with their Facebook account.
2. post comments with Facebook functionality such as linking and photo sharing. with an option to make the comment a user's FB status, possibly via a "make this my status" button or somesuch.
3. have FB photo show up instead of an avatar. (And update accordingly)
4. Comment replies would show up on users FB profile and be received as notifications.
5. LIKE your posts, photos, or other comments.
I'm sure there are a million other pros and cons to this, but one prominent advantage is not convincing users to create yet another user/pass for your WP site.