What Discourages Users :(
Users are a very sensitive part of your forum, but users are the life of your forum. By this I mean to keep your members happy.
Most members get discuraged very easily, when a user gets discuraged they move along and leave your site.
Users get discuraged when they make a support thread in a specific forum, and their topic is flodded away by spam or other post. When a member seeks help in a forum, its mostly because he doesnt know anyone that can help. So if you have a forum (Specially a support forum) when someone asks a question or seeks help, post in their thread and help them with what YOU know about that subject, that will make other people see the thread and post there.
Guest get dicuraged when they cant get to see the index page unless they register. This is a very bad technique in my opinion, because you cant get a member to join if he does not know what the forum offers.
This is just a little tip :)