Language, history and curiosities in games.
For me the test is “does this rule make me want to play this game right now?”
I play games for a living. Writing rules is what I do for fun.
If it would annoy you in an email, don’t do it in your rules.
It’s tough to read a rulebook over and over and only see what is actually there.
A mediocre game with great rules is still mediocre. But badly written rules can drag a great game into mediocrity. Time spent polishing the rulebook is time well spent.