The threads don’t "just disappear", if you use the "view new topics" function then you only see the threads that are current i.e. some one has posted something in since your last visit. Same with the listings on the forum, the fresher the thread the nearer the top of the pile it is. If you want to view an old thread you can do one of 2 things:

1) use the "search" function which can prevent different people posting the same question 1 000 000 times

or

2) Use the "view threads active in the last x days" box when you click on a particular forum, either way the post will still be available. If you want to bring it to the top of the pile, add your new, exciting and previously un-posted thoughts to it !

Threads are very rarely deleted, and offensive posts are more likely to be edited or deleted by a mod than the entire thread its self. We recently had some one delete there own posts in another section however the mods have no control over that (other than closing a thread).

Freedom of expression = good
Offensive posts = bad
Censorship = bad but very occasionally a necessary evil !

I have never deleted or edited any one since being a mod and that reflects on how cool the people who post on this forum are, however… I will if I think it is necessary !