how do you justify a game its bad or good ?

Story = 40% (including companion and RPG elements that allow me a consistent voice).
Gameplay = 40% (specifically a deep combat system with varied enemies to keep things fresh).
Graphic = 15% (nice graphics are...well, nice...but I can concede a bit if my first priorities are top notch).
Other mix issues (bug & other) = 5% (As long as these are not game-breaking I can put up with the occasional bug).

EDIT: Replay value is another big one...whether following different permutations of story elements or different combat mechanics (ideally both).  This usually comes with the first two priorities on my list.

Also 3rd person combat mechanics is pretty much a must for me as 1st person just messes with me.

I would probably rename this thread to something like "what core elements do you value that increases your likelihood of an enjoyable experience" (I know, a mouthful).  It doesn't necessarily follow that a game lacking my preferences qualifies as bad...or is good because it meets my requirements.  Beauty, after all, is in the eye of the beholder.

Does it have replay value? Replay value, at least when it comes to Bioware games, is what tells me the game is good.

I wanted to say "If a game frustrates me", but that is mostly my own fault and not the game's.

examples:

Resonance of Fate - the story looked very interesting, but I never got over 2 hours of playtime, because I didn't get the combat. (it's some weird live action/round based hybrid that if you don't choose your action in X time, the enemy gets the turn)
Airscape: The fall of Gravity - just too hard for me

I don't think that that makes those games bad, just that they aren't for me. 

I'll forgive almost anything if the game has interesting or original ideas, or if it manages to somehow be incredibly fun even though it shouldn't be.

Did I enjoy myself [y/n]?

You just know, don't you?

It's like how do I know Gimme Shelter or Going Underground or Protect Ya Neck are killer songs - there's no scientific formula.