What makes good games?

What makes a good game to use in class? This is half rhetorical (as is becoming typical). I have a few ideas, but I'd love to hear what you think.

Here are some guidelines I like to use:
1. The game needs to involve everyone most of the time.

2. If the game is for practice, the time spent practicing math skills during the game needs to be roughly equal to or more than the time spent doing the game mechanics.

3. The game needs to be fun.

4. Competition helps, so the game needs an element of it. (Caution: competition can turn into a monster!)

5. The simpler the game, the better. The cheaper the better (since it is disguised as "fun" some schools aren't willing to pay for supplies.)

6. Ideally every person can contribute to the success of the team throughout the game. People who struggle won't ruin it for the rest.


What do you think? What makes a good game to use in class for practice, review, or learning?