Lack of interest Log in with external services (Facebook, Twitter, etc..) open in small popup.

[rafass]

Actually, when you log in via Facebook, you leave completely the forum and go to facebook, there you will see your notifications, (if any) and that could cause distractions to the new users in the registration process. (because those notifications there)
much better if registering a new account just open a small popup to avoid those distractions. I've seen a lot of sites use those popup's and there you can't see your notifications on facebook: