Reviewing is a writing exercise. The more you review, the better your own writing will become. However, you should want to help out the author in your review and this can be done by pointing out any grammatical errors, continuity problems, etc.
There are five key points to writing a good review:
1. Honesty. Honest opinions are what help writers improve. Giving false feedback doesn't help anyone, and can lead a writer down a long road of bitter disappointment.
2. Encouragement. Reviews should be encouraging. Everyone at every level should be encouraged to continue writing.
3. Respect. A reviewer flaunting that they are better than the author they're reviewing is not respectful and is counter productive. Always remember the author is a person just like you, with feelings just like yours. If you don't want reviews that say, "You are terrible and need to find a real hobby," then by all means, don't write reviews like that. Constructive criticism is allowed and encouraged here. Flaming is not.
4. Well rounded. A good review should cover both the positive and the negative aspects of the fanfic. I find it easier to start out a review by telling the author what you liked and then gently offering some constructive criticism.
5. Rating. You can rate fics if you like, on a scale of 1 (error-filled and generally bad) to 5 (amazingly good). Be honest when rating a fic.
Keeping in mind the five points highlighted above, a review should contain your opinion. Grammatical, typographical and other errors can be included within a review, but don't forget to tell the author how the piece made you feel. Give him or her your thoughts about the inside of their writing, not just the outside.
Think about the fic for a moment before you review it. Ask yourself: Did the plot interest you? Were the characters believable? Did the story fit the time, place and other setting characteristics? Is there anything you would change within the story?
Incorporating these thoughts within your reviews will expand your own analytical skills allowing you to better analyze your own writing. Whether the author agrees with any of your suggestions or ideas is not relevant. You have given them another perspective on their work they would not have otherwise had. They may hear the same thoughts from a number of different people which may give them a better understanding of their readers as a whole.