Definitely an interesting idea you got there Nova, and I like your take on rating psychology. If you don't mind I'll try to give my view on it.
The issue about making exceptions and rating criteria for certain movies (one that has been discussed briefly
in this topic), is that fundamentally by narrowing something down, you remove some of the receiver's freedom of choices. Making machinima ratings for machinima movies only would force me to rate PvP movies solely based on player prowess, and neglect the aspects of presentation that are still vital to any movie, regardless of genre. Giving a PvP movie top grades based on the speed of reflexes is like basing the purchase of a Ferrari on the amount of horsepower it has - there's more to it than that.
Then comes the issue with cross-genre movies. When you think about it not all movies are pure PvP or pure machinima, but have elements from other genres in them - elements that you as a rater need to be able to evaluate. And for that to work, you need more exceptions, more rating criteria, and ultimately a bigger headache in giving your verdict. It may be more representative for those who care to take it more seriously, but unfortunately this specie amongst WCM users is a rare one, and the efforts made by these individuals easily drown in the swarm of win/fail grades.
How could we counter these two aspects in any other way than to simplify things?