Are there any topics I can't or shouldn't post?
While there aren't any topics that are considered verboten here, there are some topics that have been argued to death many times over which people have become sick of. These topics should be avoided since they no longer generate much meaningful responses anymore. Examples of such topics include Batman vs. Spiderman, Superman vs. Thor, and generally most fights involving Dragonball Z.
Also, any topic that can potentially turn into a flamewar, or includes content that is inflammatory or baiting another poster, is also fair game for being closed or deleted.
The only topics that are strictly forbidden are those regarding active religious figures. Jesus, Buddha, etc, are considered out of line in Rumbles.
Addendum: This applies regarding the religious figures themselves, not their representation in popular media.