Have you seen the Map Foundry Gallery? It's pretty spiffy. It doesn't have categories, but you can find out more information about different maps: http://wcforums.org/
Right now, the foundry is working on creating a tool to sort maps by complexity.
Gus, this is being sorted as we speak. nobodies has this in hand but here are the links to a couple of topics related to this. Map Tags
. Map Complexity
This is all going towards the things being asked for here so anyone that has an opinion, should discuss it there.
Here's one guy's opinion from way back in 2008, but we've added quite a few maps since then: viewtopic.php?f=127&t=62255