Language, history and curiosities in games.
Nobody reads a twenty-page rulebook before turning on their game console.
If I can’t have both I still prefer something that’s easy to reference. I only need to learn the rules once, but will much more often need to look for a specific rule during actual game play where speed is of the essence.
I am sure that a lot of games (and wargames in particular) would be played by a lot more people if they had better written rulebooks.
Well designed rules make for an easier to read rulebook!
My experience is that if the rulebook is a chore to read then the game is likely to be a chore to play.