Would you consider using a content locker on your main forum or sub forum so as to block viewers from gaining access without joining the forum? What type of content would you use a content locker on?
This^ - Usually all of my forums are open to guests for viewing, since it does help out SEO in the end. Staff boards are blocked of course, however I never have blocked premium boards. Everyone could view what content was on there (since my premium boards was more for downloads and what not, (not nulled content...)) and that helped out SEO as well.
I don't think it's ever a good idea to have a content locker, it would mess up SEO and guest impressions big time.
Agreed that this is very bad for SEO. I think it's better to offer different types of incentives other than just being able to see content. After all, sign ups themselves have very little value. What you want is members to post.