Ooh, that's a tough one. Generally I know if I think a game is good when I play it, but it's always hard to determine why exactly I think it's good...
Well, I'd say the reasons why I think Guild Wars is a good game, for example, is the (almost) lack of a grind (level cap at 20, and you'll reach that long before you reach the end of any of the four storylines) and the complexity of the skill system which, whereas you can only take eight skills with you, is also very flexible since you can change your entire build whenever you're in a town or outpost (respec'ing is so old).
To change genres, the reason why I thought Bioshock was good was because it combined an interesting enviroment (Rapture is beautiful) with solid gameplay (plasmids that allow you to set trash cans on fire by pointing at them, gene tonics that make you invisible if you stand still for a few seconds, and weapon upgrades that prevent your machine gun from going places and make it actually hit what you intended to hit) and a good story (that got a bit weak near the very end of the game, but oh well).
Another shift of genres, Spore is a good game because it has user-friendly creators that still allow you to create just about anything you can come up with. Unfortunately, Spore is also a bad game because the actual gameplay is so shallow. I find myself mostly creating things in Spore (especially the Adventure Creator added by the Galactic Adventures-expansion is great), and not playing the actual game very much (except for Adventures created by other people).