I had trouble getting my tags right on a post. Apparently there are four 'primary' tags at the moment, exactly one of which must be selected. I tried working with other tags only and got very confused. It seems that if you choose one of the four primaries as the first tag you choose, it 'just works'.
Maybe this needs to be clearly described in the faq. On the other hand, while I can deal with highly technical details in using a computer, I frequently have trouble with standard UIs. If it claims to be "user friendly", I will probably have trouble with it.
I am proposing posting of 'recipes' ...