everything in life is relative, so is game quality, so why bother investigating what game quality is..?
Jeeze louise.
So how about something more concrete - such as the AI's ability to handle all the situation's that may appear in a game. I would call that a quantifiable part of 'game quality'.
Then there's the interface's effectiveness.
Graphics quality could reasonably measured by looking at current, similar games.
Point is there's any number of factors that could be measured reasonably objectively before getting on to more 'subjective' factors of 'game quality'.