Here's my advice:
1. Get to the point quickly. Write the introduction like you would write the thesis or abstract for a research paper. Let the reader know if the game is good and briefly explain why. Then expand on it in the body of your review.
2. On a related note, keep personal stories out of the review. I could care less whether the reviewer grew up playing SNES games or whether they enjoyed or disliked past entries in the series (I often times see reviewers use this as an intro).
3. Be fair and balanced (easier said than done sometimes). My main advice would be to not exagerrate how good or bad a game is- just be truthful.
4. Related to the first point, I prefer reviews to be structured. Once again, get to the point and be clear and concise. Discuss different elements separately. I personally prefer sub-heading although that might be asking too much given the length of reviews.
5. Let the readers know why the game got the score it did. This is why I honestly like sub-heading in reviews or atleast a recap at the end of the review. If you give a game a 7.5/10 in the gameplay department, let the reader know why it got that score. Otherwise, the scoring system just seems arbitrary.
That's all I have right now. I also agree with what other users have posted, particularly about keeping spoilers out of reviews. This is one of the reasons why I have actually stopped reading most reviews until I actually beat a game, at which time they are actually useless.
