Another problem with a rating system...
Often one does not know how bad a GM was until much later. Speaking more from my experience in LG, the default thought is its a rushed, poorly written module. One talks with friends about adventures played or ends up reading the module to judge it only to find it wasn't a bad or killer module but a GM who flubbed the whole thing up.
The converse was often also true. A GM looks bad because the module was spotty.
