I think all ... i think it is like you say if the content is good then they will register and join in ,
That's just my opinion from visiting other sites that say "you have to be a registered user to view" and all i do is click off the site and look else where.
Hiding signatures or other elements isn't a bad idea. But yeah, if guests can't see anything then at least from my experience, they won't even bother unless they're desperate for something, or extremely interested.
i don't let visitors access the attachments that are posted ... so if a new visitor reads what some one has posted and then wants to download the attachment this gives them a push into registering , i usualy put a small caption in the post just to let visitors know that they have to be a registered user to access the attachment
That's too admin mindset - "block important content so people will join to see it". Think of it from the user's perspective. Would you sincerely join a forum if they block viewing of the content? Probably not. You wouldn't know what to expect, so I see no purpose in signing up to a forum not showing content when there is the next link down in the search engine which content is shown (that's if you can even rank well from not allowing content to be seen).
Sure you can disable such as premium tutorials etc. and only registered members can access them, but there's no successful logical reason to disable viewing of content.
If you're visitors aren't converting into members, you need to offer some incentive, not disable reasons to visit your forum.