If you want to learn, check out HTML Hints. All decisions are made by the forum administrator. Anyone can participate in policy discussions, no matter how recent their arrival. If you are worried about doing something that might get you banned, chances are you won't ever have to worry about it. As the RoE mentions, the inventiveness of a small minority forces us to keep our options open.
All threads have an assigned host.
In focused threads, they also set the discussions. In general threads, they have less of an immediate role and often may do nothing other than participate in the thread discussions. A thread host can delete a post for any reason at all. If the forum administrator determines that a thread host is abusing this privilege, he or she will be removed from that position. Most thread hosts have deleted only a few posts.