I wanted to ask how the folks here view the rating system. I ask because I get the feeling some folks are rating games 4 and 5 stars simply because they like a game. That's fine and all, but the point of the system is to help other people get an idea of a game's quality. Therefore I suggest we try to stick to this -
1 stars - Crap. Avoid at all costs.
2 stars - Mediocre at best. Nothing to write home about.
3 stars - A good game. Worth playing at least once. Most games would fall into this category if they are good or 'okay'.
4 stars - A great game. Recommended and well worth your time. This is reserved for games that are really good, but not quite 'classic'.
5 stars - Classic must play title. Games like Fallout that you feel everyone should play. Chances are games rated this high will be loved by the majority of gamers or fans of the genre in question.
The reason I say this is because I personally would rather not see 'okay' games rated 5 stars. When you start seeing this it becomes pointless to even have a ratings system. Obviously the written reviews should be used over a simple score, but it's still nice to be able to get an overall feel for the game.
Obviously it's very subjective, but if a game has 5 stars and 100+ reviews chances are it is either doing something very right or people are over-rating games. I mean we have 5 stars for a reason, and it would be very sad if every single game on GOG was 4+ stars because I sincerly doubt they all deserve it.
Earthworm Jim 3D for instance is a really fun game, but really - is it worth anything more than 3 stars? Should Shogo really get 5 stars and be a "must play" game for everyone along the lines of Fallout and Freespace?
I would rate some of my favourite games as 3 stars. I love them, but really - I'm not going to recommend them to every gamer. While their shortcomings might not bother me, I should be well aware of how they will bother others.

I'm experiencing Deja vu, there have been threads like this before.
However, I think I like the ratings here instead. But mostly, I just rate a game on my own personal preference. Just because X game was really well done, really well thought out and etc, it doesn't mean it'll speak to ME, but I might be able to see how others would enjoy it more than I did. Sometimes there are games that just don't appeal to me, no matter how well thought out or designed they are.

I think we'll end up with more realistic ratings when we get more members, and it averages out.

If so then I apologize, I looked but obviously not hard enough.
Everyone rates on personal preference. We can't help it. I'm certain there are folks who think Fallout is a crap game. However I guaruntee the amount of folks who love it outweight him/her and for good reason too.

I agree with your definition of each rating, but there just isn't any "solid" one. Though, I guess it might help if GoG.com had a kind of official definition, for example by having a description like yours visible on the page where people can write about the games.
But maybe that just isn't necessary. After all I guess everybody will agree that the actual texts describing how people felt about each game are far more important than the rating, as that just can't express nearly as much. And so far I have been pretty content with how people did that, mostly not just writing "Yeah, great game!" but really going into detail. That is what really helps when deciding if you want to buy a game :)

I think the main issue might be that people are too used to sites like IGN or the latest gaming magazine that use a percentage system. The percentage system is judged like you would grade a paper in school. Thus you rarely see anything less than 70% unless it's a steaming pile - which I feel is not right. You get too many games in the 80's and 90's that really don't deserve it.

Nothing between a good game and shit game?
I rate like this.
5 - Outstanding.
4 - Very Good.
3 - Good. Fans of the genre will probably find it even better.
2 - Mediocre. Fans of the genre could have limited fun with it, but not worth a full-priced purchase.
1 - If you find it in the bargain bin, make sure to bury it at the bottom to reduce the risks that someone inadvertedly picks this up. In other words, shit.
I also feel the ratings on GOG should be stricter than the gaming industry in general. They're all older games, and there's no censorship here. On sites like GameTrailers every crappy release out there gets a 60%+ score, which is idiotic.
A game scoring 60% here should qualify as a '3', or a decent game that fans will probably enjoy. Five stars should be reserved for the truly outstanding titles that nobody should miss out on.

Stonebro's system is pretty much spot on.
Once you get to the rating of 'fun only for those who like the genre and may get some fun out of it' then all that's really left is 'stay well clear even if you like the genre because you will want to rip your eyes out and blend your brain into a mush to forget you ever played it'.