Language, history and curiosities in games.
A game is ready when all players want to play it again and again.
When all players say the game is very good, improve it.
Good playtesting sometimes hurts.
It’s extremely dangerous to test with people who want you to succeed or who like you or who are just too nice. […] Playtesters should be trying not only to see if your game is fun but also to break it wide open.
For me the test is “does this rule make me want to play this game right now?”