It would indead be nice to have some guidelines. Most of the time a give a hint to the person who started the question to accept an answer as the best answer. But in the example you stated the user found out it was a bug of his own and that he had found the cause. Since he can not accept an own answer as the best answer I closed the question so it is visible that the problem has been solved. Most users are not aware that they can close their own question (or reopening the question again if something pops up again). It would be nice if more users would close their questions.
As a moderator I can delete answer that are really a comment but it is not possible for me to change it to a comment. That would be a nice enhancement. But if the rule above was consistent this would not matter because it would still be visual that no correct answer has been given yet.
I think it would be nice to change the dark green colour (when an answer has been given) would be changed to orange and only when the question is closed or a best answer has been given it would turn to green. Would give a much better visual clue of the still open questions. There are a lot of questions that never got a definite correct answer and just disappear of the main page (very nice though that since a couple of months we can now go the the older questions, thank you Mendix for that).
While we can offer guidelines for expected forum behavior, it's completely up to the individual users to follow or ignore these guidelines. And we'd like to keep it that way, because that's the freedom we want you to have. You shouldn't feel forced to do anything specific when using the Mendix Forums.
The way it currently works, is that the more active/experienced/veteran users, who have enough points, get a little more options. How they enact that power is up to them. Power to you! Of course, if we see things going horribly wrong, we'll intervene ;) So far it has worked out pretty okay I think.
What do you want to achieve exactly with these guidelines? We can't force users to mark answers as accepted answers, or to reply when someone asks for more information.
The goal of the forum is for everyone to be able to ask and answers the questions they want. What do you want to change to improve this?