More categories or more specific categories in the category tree. Example: if I want to see only sngle engine planes. There are plenty of great maps on the site, but making them more easily found would help. Some of the categories seem to general for the vast amount of maps in them. The street view "Vehicle - cars and trucks" has many, but I only want sports cars, for example. Or even the animal category, maybe only cats. I'm sure they can all be found but the tree you have is nice and I believe it should be expanded.
Closing stale issues
I agree with the idea, though the micro-management of the categories worries me. Moderators end up spending a lot of time ensuring posts are in the right categories (which is subjective) and moving maps around as we re-organize categories.
Would it be useful at all to have the community be able to add 'tags' to posts? Eg, you could tag a 'cat' in the animal category, or 'single engine' in the airplanes category.