One possibility (not sure if it's been done) is to try to understand what the players themselves think of the game. Some WoW players love it, some kind of hate it but continue to play it anyway for various reasons, ranging from addiction to sunk cost to social pressure, others vaccilate or are somewhere in between. How does that compare to FarmVille players or to Starcraft 2 players or to slot-machine players?
Self-assessment isn't a perfect metric either, but differences in self-assessment between players of different kinds of games might give something interesting. I mean, if you go only by money made, the slot machine is still the best game of all time. Which maybe is true on one axis of "best", but it seems weird to argue that it's the only possible way to discuss games. Is looking at the top-grossing films list the only possible way to discuss films?
My point is that if you want to evaluate quality you have to leave metrics behind and start looking at opinions. And opinions are like assholes in that they're ubiquitous throughout the population.
Of course you can make up metrics based on averages of everyone's opinions, or weighted averages where you weight by the "sophistication" (in some sense) of the person's opinion. But nobody will believe such metrics anyway, in that you'll never catch anybody saying "I hate this game/book/album, but this reliable metric says it's good, so I guess it must actually be good!"
If Farmville is bad, it falls into the "fascinatingly bad" category of things that are derided by professional opinion-makers but still vastly popular. Genuinely bad games, books and bands are a dime a dozen -- check out a publisher's reject pile, an open mike night or a twelve year old programmer's hard disk for examples -- but fascinatingly bad things like The Da Vinci Code or Nickelback or Farmville deserve a lot more analysis.
Self-assessment isn't a perfect metric either, but differences in self-assessment between players of different kinds of games might give something interesting. I mean, if you go only by money made, the slot machine is still the best game of all time. Which maybe is true on one axis of "best", but it seems weird to argue that it's the only possible way to discuss games. Is looking at the top-grossing films list the only possible way to discuss films?