- Don't swing at the first pitch after back-to-back home runs
- Don't work the count when your team is up or down a lot
- When hit by a pitch, don't rub the mark
- Don't stand on the dirt cutout while the pitcher is warming up
- Don't walk in front of the catcher and umpire when going to the batter's box
- Don't help the opposition make a play
- Relievers take it easy when facing other relievers
- Follow the umpire's code when addressing them on the field
- Pitchers stay in the dugout at least until the inning is over when they are removed
- Pitchers never show up fielders
My June 2009 post on The Economics of Sportsmanship explores why these kinds of rules exist. Whatever their origins, such customs crisscross all corners of "baseball society" -- the "ten" include actions directed at teammates (9 and 10), umps (5 and 8), and mostly opponents (the rest).
These non-codified codes, while sometimes individually very narrow, matter for the smooth functioning of the game. The undercurrents of how such rules come to be and why they matter parallel unwritten rules in society as a whole. A Cambridge University Press book, Violence and Social Orders (by North, Wallis, and Weingast), examines in great detail governmental rules but also societal customs in determining the degree of "openness" of communities and nations and how they have mattered for economic well being.