What is the one feature FriendFeed must have to go mainstream? E-mail. FriendFeed must have some sort of notification when someone comments on anything you post. Obviously, this should be a feature you can turn off, but people will continue to think FriendFeed is too complicated until this is in place.
Facebook has this currently – any time someone comments on a status update or anything I post, I get an e-mail from Facebook with the comment right in the e-mail, giving me the option to respond on Facebook. FriendFeed already does this, sort of, if you haven’t been on the service for awhile. However, there is no way to get an e-mail on every comment sent for certain posts. People are used to this, and as long as you can turn it off, it will not be construed as spam. If FriendFeed is to compete, they must support e-mail, on every post you send to the service.