The first thing you do when you build a map is make the floorplan. This dictates how the map will eventually play. Imagine a wire-frame model when thinking of the floorplan. Themes can be powerful in making someone download your map, but it doesn't make it great unless it's done right. This could be anything from teleporters that don't teleport, to flags that don't capture.
Bugs also covers bot pathing, since nobody likes to fight bots that just stand around.
Hopefully each author has taken the time to thoroughly test their map and eliminate bugs. A bug free map gets a perfect score of 10 in this category. And even those of us who do download a map that slows it down every now and then. The better the map performs, the higher the score. Here's where we rate the flow and consistency of the map.