I agree with having a new type of game rating system, especially since MMO's are much more complex and re-playable. In fact, I think it should be much more elaborate and have different categories for every AG game like:
1. Controls
2. Soundtrack
3. Visuals
4. Storyline
5. General Gameplay Satisfaction
6. Would you play a sequel of this game? (A yes or no)
I think that having more elaborate statistics of different aspects of each and every AG game will make users more aware of each up and down of the games.
As for MMO's, since most or all (I am not sure) are owned by other sites and AG is simply hosting them, maybe AG isn't implementing a ranking system for them due to this kind of issue.