I sincerely believe that making people register on a forum is difficult; and a big registration form makes it even more challenging. There are people who do not want to use FB/Twitter/Google to login and I think a simpler and intuitive registration process can help forums gather more members.
Limit the registration form to Email Only! The flow would look like this:-
1. Display this email field in the sidebar or on a separate registration page or on the top slide-down panel like this:
[ Enter Your Email Address ]
[Confirm My Registration] <- Button.
[Social Login Buttons, If Enabled]
2. User then gets an account activation link.
3. Clicking on the link takes the user to a customisable, post sign-up page. This can have customisable sign-up page:
- Type your desired user name: [ ...... ... ]
- Type your password: [......]
1. You've got their email address, baby. You can send them one or two follow-up emails, if they do not complete their registration.
2. You saved your visitor's time and efforts in filling up large form.
3. User thinks: Wow, sign up is so easy! I love it!
4. You get more sign-ups.