This is both a test of how to use a wiki by doing, and an attempt to propose some basic ideas about game design. For now, I'm just going to list a couple things:
The first consideration is playability - not how playable "I" think it is, but how playable others find it. Playability is not the only consideration, nor necessarily the most important, but my belief is that it should be the first consideration.
By this I mean making each part or aspect of any game as simple and easy to understand as possible: simple words, simple sentences, simple examples, and simple steps from one point to another. Game play that is very complex can be presented in a simple, easy to understand manner, the simpler the better.
This is enough for starters.
The following was added by DavidHowe:
Here's a summary of what Mark Thompson feels makes a good game:
In essence, how many different levels of expertise could the game support.
How easy is it to form a judgement on how good a particular move is.
How possible is it for a player in a weaker position to recover and win the game.
How possible is it for a player to achieve a position from which it is impossible for the opponent to win.
I guess these would go under the "playability" consideration. In any case, for the full text of Mark's essay, see the page on his site: Defining the Abstract.