A danger here is that a new user makes a typo in their e-mail address when signing up, puts a fair amount of effort into their new profile/account/whatever, and

A danger here is that a new user makes a typo in their e-mail address when signing up, puts a fair amount of effort into their new profile/account/whatever, and is then locked out forever. They won't realise they made a typo and will blame the site instead. But I agree that new users having to leave the site to check their e-mail is a hurdle to get over.