I think most of the time people are generally rating a thread 'because' of a map.
That was how i always viewed it. I'm not sure that there is a map-specific rating.
People rate the thread and rep the map maker... but I don't think any of it can be tied directly to a map itself.

edit - though maybe it should be done as more map specific or tutorial specific.
Maybe that's something that can be dealt with at some point, if that's something people are interested in or concerned about.