There's an awesome article by Joe Grundy over at BGG about the accuracy, interpretation and resilience of the game rating system. By coincidence I went through and rerated some of my less favourite games yesterday after thinking through what those ratings really mean. The BGG ratings are given minutely short descriptions by Aldie which causes problems - 6 means "will play sporadically if in the right mood" and 5 means "slightly boring". I find many games slightly boring but struggle to think of games which I'll play if I'm in the right mood, so I would tend to give 5s rather than 6s. In an ideal world the sum of all BGG ratings would approximate a bell curve, but if everybody liked the 5 description better than the 6 description, as I do, then the bell curve would be warped. In theory, Aldie's descriptions can change the shape of the curve :-).
Consequently I have my own interpretations for the BGG ratings, and I admit that my interpretations aren't guaranteed to produce a bell curve either. But they're close enough for me.
10: Always want to play this and expect it will never change - "NEVER" is a long time, so the only game I can confidently rate a 10 is Scrabble, because it's been 30 years or so. I rate a few other games a 10 because I'm there must be some others, but they might change.
9: Awesome game and I'm always excited to play it. This is about the same as what Aldie says.
8: Very good game. I like to play. An 8 is my default rating for a game that I liked after 1 play. After I've tested the game a couple of times, if I really like it, it might go up to a 9.
7: Good game, usually willing to play. If I play a game and it's decent but I'm not really excited about it, I will initially rate it a 7. If I keep playing it and discover I don't like it, it will fall, but it might climb as well.
6: Games that are just alright. For example, Carcassonne holds some attraction for me, but it's not very interesting.
5: Games that I don't care for but I might one day want to play them. Louis XIV and El Grande are here because although I really dislike area majorities I can see that it's a game mechanic which makes sense.
4: Games that I wouldn't care if they vanished off the face of the Earth, but I might play them one day anyway. I dislike them but I might want to give them another chance or can see some little value in them. Power Grid is in here.
3: I would actively argue against playing this game or even outright refuse. This is where I rate real games that I really really hate. For example, Australia, Andromeda, Railway Rivals. Don't get me started.
2: Yes it's a game, but it's no fun. A game which is so little fun that I won't ever play it again earns a 2.
1: This does not count as a game because I cannot imagine having any fun with it. Very few games are technically broken, though some which are such as Betrayal at House on the Hill are still fun anyway, so I use the 1 rating for games which couldn't possibly be any fun.