Language, history and curiosities in games.
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?”
It is vital to remember that ten thousand fans will play a game more in a single day than ten designers will in six months. They will find ways to play that the game’s designers never dreamed of.