This can be compounded by the fact that most publications give out annual awards. For example, let's say Generic Game Magazine gives Quake III Action Game of the Year. A freelancer reviews Quake III's expansion pack and in the course of the review states that Quake III was a pile of crap. For obvious reasons, this is unacceptable -- it contradicts the magazine's established stand on Quake III and will erode the magazine's editorial credibility. This comes back to the earlier point about knowing the publication, but too many freelancers simply don't take the time to do their homework before writing a review. (And by the way, the above example is based on a real-life incident, although the game wasn't Quake III.)
As a freelancer you must realize that when you write a review it's not your review -- it's the publication's review. If the game gets a bad review it's the magazine that will take the heat from the publisher, not you; and the same principle applies to games that get high scores. Take Half-Life, for example. Colin Williamson reviewed it for PC Gamer, yet when the review is quoted on the game box it credits the quote to PC Gamer, not Colin Williamson.
Because of this, editors are very, very protective of review scores. If you constantly turn in reviews that give every game a high score, the editor won't trust your judgment. If the body of your review consistently doesn't back up your score they won't trust your judgment. If they get a sense that you're not playing the game through to completion they won't trust your judgment. And once you've lost that trust you're on very thin ice professionally, especially since most editors are friends (or at the very least professional acquaintances) and they do talk to each other about freelancers and how those freelancers conduct themselves.
Act Professionally: This applies to both your dealings with the industry and with your interaction with an editor. Do not call an editor every ten minutes looking for work; when they have work they'll let you know. The more you harass an editor the faster you'll find yourself without work. And if an editor asks you for a rewrite, take the time and properly rewrite it per their specifications. Your job does not end when you turn in the review, it ends when the editor signs off on the review.
When dealing with game companies, be professional and courteous. It's not cool to get in a fight with EA's VP of games at E3. Editors do not hire people that have a rep for being difficult because they don't want those people representing their publication. (And when you're working on a freelance assignment, that's exactly what you are doing.)
Life as a freelancer can be very rewarding and a great deal of fun. The bottom line, though, is that you must conduct yourself as a professional at all times. Keep that in mind and you'll go far.