I'm not sure if there are any set-in-stone rules, but some general guidlines (ie. things I'd delete or close threads about) are:
1. Don't flame people.
2. If it's illegal, don't talk about it here - this includes where to obtain pirated music or software.
3. No graphic language, links to porn, etc.
4. No rascist/sexist/homophobic/etc. comments
Some things that are ok to say but wouldn't be met with a very positive response:
1. Attacks on JRiver or their software. You can complain about a problem you're having, but keep it polite and provide information about the problem you're having. Don't say things like "This software sucks!" or "Support here sucks!"
2. Requests for help using competitor's software.
I can't really think of anything else that the moment. As always, JimH (the CEO of JRiver) has the right to shutdown any thread for any reason at all. Sometimes he does so simply because discussion of something harmless - but unrelated to MJ/MC may clog up the forum.
Don't take it personally. We've all been deleted from time to time. I can't speak for the other moderators, but I know I have. Sometimes you stumble into questionable territory simply because you didn't know about certain behind-the-scenes developments at JRiver.
Anyway, hope that helps.