A review is a critique about a game that you've played (Yes played. It is extremely rare for a review based on watching a long play to be good). Based on that, it isn't a requirement for you to know the game 100% or even having finished it. Just a basic understanding of the game. A review will go over the main points of a video game and point out what the person thought was good and what he/she thought was bad. If you've ever taken an English course then you would know that backing up your claim is the critical point of all reviews.
What isn't a review?
A review is not just slapping your opinion about the game. You have to answer the big old question of "why" it is what you think it is. Your audience ranges people that may be hoping to buy the game to people who have never heard of the game before. It is critical to make a balance of making the review easy and fast to read while giving enough details to give the reader a general idea about the gameplay, storyline, and other critical components of that particular genre.
So here's some common rejection reasons:
1. Formatting is bad!
Formatting is fairly simple but you need to keep it orderly. Remember to use paragraphs and to use two line breaks (for a double space) between paragraphs. Also remember to check your grammar and spelling. One last thing is to check that your bold, italics, underline tags are proper.
Rule of Thumb here: If it looks like a hideous gigantic wall of text, it's not going to get published.
2. Lack of Content
This is a big one. The problem is this:
"I like this game. It is very good and I had lots of fun. The storyline is very good and the gameplay is fantastic. You should buy this game."
This is a generic summary of your opinion. It isn't backed up with any kind of reasoning making your credibility go down the drain. Why is the game good? Why did you find the gameplay fun? Why would I enjoy it like you did? This doesn't give me any kind of image to what this game would be like.
Another common problem is completely forgetting the gameplay (the heart of all video games). What happens when you forget this part is that your review doesn't tell us what kind of game you're reviewing (you could be telling us about an RPG, FPS, RTS, we wouldn't know). If we can't tell the genre of the game from your review, then you should probably revise for better content.
Rule of Thumb: Most reviews that are this are extremely short. While we're not expecting a novel, we do want something more than a single paragraph (if your single paragraph is a giant wall of text then you have a formatting problem rather than lack of content and need to see point 1).
3. Your review looks like an FAQ
FAQs are good and if you want to write something with extreme detail, you may want to consider writing an FAQ/Walkthrough to the game instead of a review. While it is necessary to give a general idea, only give out enough to support your opinion and make your vision see-able by the reader.
Rule of Thumb: Problems of this type usually show up as lots of lists about things in the game. No need for that much detailed info.
4. Useless Information
This is the wrong way to solve point 2 in this list. Don't just throw out random stuff to put more fluff into your review just to make it look like it has lots of content.
Rule of Thumb: Make what you write relevant to a game review. Also do not restate your points over and over and over again (this is bad filler).
5. Review takes negative to the extreme
A negative review is okay when it provides constructive criticism. The best way to do this would be to compare the game in question to another game that's similar (or at least in the same genre). Don't take comparison's overboard though. Spite reviews are not informative. Remember that it is extremely rare for a game to be truly be bad in every single aspect (example being Big Rigs).
6. You submitted something other than a review.
This is the most common reason that reviews get rejected. Please stop submitting requests to update trainers and membership questions in the review slot. There are special sections of the website for these.
Spoilers in your review is probably one of the worst things you can do. If a reader reads this information, you have effectively ruined the experience for him/her. Leave spoilers out of reviews.
8. Improper Language
Please do not submit reviews that would be similar to AVGN. Keep it professional.
9. Game has not been officially released
This tells us that you illegally obtained a leaked copy of the game, breaking a non-disclosure agreement with the developer, or the game is in public beta. Either way, if the game does not exist, content for it will not be published.
And that should cover some basics of writing reviews.