HOW TO AVOID GIVING A BAD REVIEW
Reviews are a wonderful thing. It offers feedback to the creator and allows potential customers to get an idea of what to expect.
However, I've seen some really bad reviews recently. I know most people don't bother to give a review because it takes a time commitment, even if just a few seconds. But believe me when I say that most publishers really value them (I know I do).
But I wanted to give some advice. I know as a publisher that you're always going to get some bad reviews, and you can't ever please everyone. Most publishers are cool with those sorts of reviews. But here are some things that drive publishers nuts that I've seen recently (in places like DTRPG for example)
* giving a low rating because you don't like the genre/theme, rather than the product itself. For example, giving a bad review to a Western themed supplement that uses B/X mechanics because "I don't like D&D to mix with Westerns". I don't particularly like the mechanics of 4e, but if I went around and gave 4e products bad reviews based on my general preferences as opposed to the actual product, I'd be kind of a dick. And certainly not fair to the people who wrote those products. Just saying.
*accusations of copyright violations. These are pretty serious accusations, and while there are people who have stolen IP not belonging to them, you better make SURE before you accuse someone of doing this in a review. I've had people give me a bad review and accuse me of stealing art from Larry Elmore before because they had no way of knowing I had spoken with Elmore previously and gotten his permission (which was noted in the product as well). I actually have several examples of things like this.
*Making it personal. Luckily this hasn't happened to me, but I've seen it happen to others. "Oh, you just gave that guy's product a 1 star and said how horrible it was? I couldn't help notice you and he got into a heated argument the previous day." This is especially noticeable when every other review was pretty decent.
*Accusing the publisher of being unprofessional because of assumptions. For example, one review stated "The maps look too good, they must have been traced from something else. Very unprofessional." Unless you're a professional yourself, might want to hold off slinging these accusations around. Especially since you're probably wrong unless you have definitive proof. I actually had to give a guy my PSD files to prove everything was legit before he'd correct his false accusation for a product once.
So basically, it's OK to give a negative review. Really is. But don't make it personal, and don't make accusations unless you have proof to back them up. Try to stick to as much objective criteria as possible. EVERYONE will find your review more helpful that way. And since you're taking the time to write one up, why not make it useful to people?
Anyway, sorry to rant.