From the perspective of a web developer, what I don't like about Facebook Connect is I give up ownership of my user accounts.

In other words (unless I ask for and maintain separate user tables), all the login credentials stay with Facebook. And if they ever decide to stop supporting my site, I lose all my users.

I'd be willing to pay for a FB Connect like service if I trusted it (which would probably mean the ability to download login info on anyone who connected, so I could roll my own user management whenever I wanted).