Define what you, as a gamer, like to see in a game. MMOs are quickly becoming like TV stations; something for everyone. With more and more choices, you have a choice in what type of game-play experience you will receive.
For me, I am looking at a game's class (or skill) balance in PvE and PvP, the learning curve, immersion factor, continuity among the world, PvP system, and most of all fun factor. It is best to start a review looking for the positives, rather than focusing on the negative.
Cover the basics. Graphics, technical performance, and customer support. These are areas I prefer to stay away from lengthy debates on. Everyone has their tastes. I'm more worried about whether the graphics, technical aspects, or customer support detract from the overall experience of playing the game.
List the negative, but keep ranting to a minimum. Provide a reason why you dislike it, offer a solution if you have one, and move on. A balanced review will encompass the good, bad, and ugly.
Tell us why you will or will not be playing the game for years to come.
End on a good note. Each and every MMO has something unique that is attracting players; some game's have more than others. Find what that is and highlight it. Don't try to run the game into the ground, otherwise you are writing the review for the wrong reasons. Try to give the reader some hope that the game will be a wise investment of their time, or give them solid reasons why it would not.
Update: 8 Jul, 2007 - Edited post and applied labels.