Some games are fun. Some games are interesting. Some games are more than that. I tried the other day to explain to an email correspondent why I like games, and I think I had an insight - games are a world in which players can interact in ways that are not permissible in the real world, and a great game is a game that supports richness of that interaction. The game is an abstract lens through which players can see each other as they could be.
What does that mean? Take Carcassonne as an example. It's often the case that I have a tile that could be played to help you or to hinder you. I can choose to be nice or nasty. In the real world, I would choose to be nice. In the game, I can choose to be nasty and you won't resent it in the real world. I can also choose to be distant from you, or in your face. Our relationship in the game may be quite different from our relationship in the real world. The game lets us interact in ways we never would in real life.
I play many abstract games (Gipf series, Gobblet, Quoridor) against by brother-in-law BIL. In those games I often find a particularly cunning move which lets him know that I've seen what he was up to and I've found a foil to it AND I've backstabbed him at the same time. His inevitable comment is "Oh, you bastard!" And he sometimes does the same thing to me. As abstracts are particularly easy to make stupid mistakes in (Gobblet has a rule that if you touch a piece you must play it), we also act much stupider on average in the game than in real life.
In a game you can be aggressive, nasty, annoying, nice, tricky, cunning, aloof, bizarre, and in general behave in ways you can't do in real life. Citadels, which I played again last night, allows for a large range of nasty and tricky behaviour which you wouldn't otherwise get away with. It's a great game, though emotions do tend to seep into real life a little.
I find two player games to be particularly good, probably because each player has more control and can express more of their personality through the flow of the game. Even Scrabble (which must be played two player to be at its best) supports all the things I've mentioned. I've been known to play a fairly poor word down the side of the board mostly with the intention of keeping my opponent away from the Triple Word Score. Nasty, but accepted behaviour. And if my opponent didn't do it to me I'd wonder what she was up to.
The GIPF Project games are great, but I find them a little too abstract to express a great depth of player emotion, which is why I haven't rated any of them a 10 despite having very high regard for them. I think you just need a little more complication, such as you find in Trias or Domaine or Citadels, to be a really great game.
I think I'll eventually explore this idea further. Opinions?