For me it's less scientific as to what makes a bad game, I've enjoyed some pretty terribly rated games before, if the game does not engage me and make me want to come back and keep playing for any length of time, it's a bad game.
There are different levels of this too, for instance RA3, while I was playing the campaign, it was a decent game, but I don't really feel the draw to go back and play it again, turning it into just an OK/mediocre game to me.
But some things are tough to overlook, regardless of mindless fun. such as obvious bugs and imbalances, lack of any challenging content, no replayability, horrid voice acting, and a game that is simply put on the market to show off pretty new graphics technologies.