I know it sounds a little obvious but I recall when I joined how confusing I felt the topic area's in that there were so many "grey areas" I was unsure which area my query fitted in.
sometimes you go into say "body and soul and you find a list of medical questions which are a bit offputting because technically they *are* body but I feel body and soul is more a "feelings" type of place rather than a "health and fitness" area which deals with mainly physical ailments.
I think the subtopics need to be permanently visible, I know this would be quite messy looking but it would open the areas up more to new questons and prevent people posting physical ailments in body and soul
maybe a walkthrough on posting etiquette and ensuring your question is posted in the right area?
Not sure if I could do that in a video, but I might be able to put something together.
Also, I think the category issue is a separate one - we're going to trim them down sooner rather than later. I think improving the categories rather than trying to explain them works best.