A good review is one where you can provide all the details of the following:
The game itself
Story (if any)
How it works (the gameplay)
Features (graphics, game modes, etc)
Pros and Cons
The more details you provide on how good or bad a game is, the better. Don't just say this game is cool. Give a reason why it's cool and why that reason for being cool is cool. What makes the fighting system in God of War 3 so great, for example? Are there more combos than before? Does the result of the combo bring a satisfactory result? How easy/hard is it to perform them from a casual player's view or a hardcore gamer's view?
Also, the game(s) you review has to be one where you managed to play towards completion (unless it's a casual game). You must also consider whether the game you review can be played by everyone or to a specific group (obviously not everyone can easily pick and play a First-Person Shooter or a Real-Time Strategy game, especially if it's someone who has never played a game before). Finally, you must be unbiased when reviewing a game, no matter how much you love or hate a franchise (Naruto games, for example).
One more thing: do not emulate how other reviewers do their games, especially those at X-Play (the writers tend to make the reviews more of an entertainment program than talking about the game itself). Do your own style, but keep the focus on the game itself.