Facebook Connect & Google Friend Connect as only way to register & login
pedalpete: I think it depends on what you are doing. If your site is something socially connected, where people are likely to want to share their activities with friends, then I think it makes sense. 
If it is more of a stand-alone thing, I think it is disliked. 
If you are doing anything in the enterprise realm, I would strongly suggest staying away from FB.Is the login functionality central to your idea? Or can people use your site without the login?At the same time, I think it gets confusing to have too many options for login as well. I sometimes go to a site and try to remember if I created my own login, used FB Connect, or OpenID or what.