Recently I lost some points to the difference between riddles and situation riddles. I didn't really understand the difference until @Emrakul pointed it out.
However, I did look through the FAQ for this site to get some ideas on how it worked, and also how to do the hide-a-comment technique (which reference I could not find; fortunately I have enough rep from other sites to sneak a look at people's code).
I suggest that we add some clarifications somewhere about how to ask and what you can ask.
We might also add some basic definitions - or references to them from the wiki - in that same FAQ page, like the difference between a riddle and a joke, for example, and that you shouldn't post jokes (otherwise the site will become junky). But there are other distinctions too that come from conclusions derived in this meta.
Right now it seems like the only way to know the "rules" is by reading through all the questions in the meta.