(1) Account registration
The new account creation page could be radically improved.
First, can you consider checking multiple errors at once? Thus, if the captcha is wrong AND ALSO the username is taken, the user doesn't have to submit a new captcha, and then a new username, and often a new captcha with that.
Is it possible to use Ajax to verify whether a username exists, to end the guessing game?
When the user has chosen a username that is already taken, perhaps instead of jerking the page view to the top where it tells them that there is an error, the screen can be retargeted on the area of the error, and the error message included there.
(2) Forum registration
When logged in to the Skype website, if a user goes to the forum, she must register there.
The forum gives no indication of registration status at this point, and while it admirably posts whatever message that user was trying to post, it also doesn't indicate that this has happened, leading to duplicates.
Hope these help!