10 : In every way, to the tiniest seam, a perfect game - a gift from the gods. I really, really don't believe one game will ever be able to hit this mark.
9 : Practically a 10, the decimals are there to make the rating more specific. You must try this, at the very least.
8 : Despite some more or less minor faults, a great game.
7 : Basic stuff; a good game, but missing it won't kill you.
6 : A decent way to pass time, but you should consider renting or borrowing it instead of paying for it.
5 : Lingers between mediocre and really bad, OR it's a sequel that carries an old flame a bit too far, and gets punished for that.
4 : A horrible performance and vulgar waste of the expensive capacity of its format.
3-1 : A BOMB. "Shit slides downwards", so the worse the rating, the further you should evacuate for the sake of your own sanity.

NOTE: Reviews written and published before January 28th, 2012 have specific ratings for each element of the game. Here are the explanations for the ratings.

GRAPHICS : General look, effects, smoothness of gameplay in relation to these effects, all-out visual design including sprites
SOUND : Music, sound effects, voiceover work
PLAYABILITY : Controls, gameplay, feel, the game as a whole - has more effect on the overall rating than anything else
LIFESPAN : General difficulty - or how long you'll want / be able to play a nearly impossible game, replay value
CONCLUSION : Usually an "average" (+/-) based on Playability and Lifespan, but sometimes it's based on my own instinct and feel of the game