I'm playing with the Sandbox for 5.1 and discovered that if you use the Business template you can't reply to forums. For some reason the Reply Box doesn't come up properly. Set to No Template and it works ok.
Bigger question, though. Is there a way to set that users can post a message (new thread or post) but have it need to be approved before it goes live? I have a client who would like to do this but who needs that feature. They want people to post but they want to vet all posts before they go live.