Those that sign-up that don't register fall into two categories:
1) Those that sign up and get their email address wrong and then wonder why they didn't get an email/sign up again with a different name
2) Those that sign up with a free email account (Yahoo, Hotmail etc)
How many of you use free email accounts and how many of you have the junk mail filter on? I'm tempted to ban free accounts on the registration page as 90% of incomplete registrations are from these users.