Lifthrasil: Well there is the fact that fans of a game are fare more inclined to rate a game than non-fans. So the star-ratings are essentially useless. But reading the reviews helps, since the good ones among them not only state that they love a game, but also why. And from that you can conclude if these reasons apply to you too.
That said: this
link here explains what these star ratings actually mean.
Heh that link is pretty funny. Netflix actually has a system where they define what each star rating is supposed to mean, but I don't think many people actually follow it the way Netflix intended. For example three stars is supposed to indicate that you liked the movie, but many people feel that three stars is a substandard rating. I actually wish that Netflix would add half-star ratings, but they said they won't add them because they've done some research that suggests most people don't need those extra half-stars. I tend to put a lot of thought into my reviews so I could definitely use the extra half stars. I often find that a movie I've watched is better than other three star movies I've seen, but not good enough to compare to other four star movies I've seen. To me there's a big difference between three and four stars, and this is where the majority of movies are going to fall into, so it seems like there should be a half star option in there.