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Big expensive books are a part of the oppressive social footprint problem that RPGs have. If you want to game with your friends, as opposed to making friends who game, you need to be able to share a game without necessarily making your friends read hundreds or even dozens of pages of rules.
So rules that you can explain in a few minutes at the start of a session, or as the game progresses, are cool. But short rules that you can print out and hand to someone are even cooler! John Harper is great at short rules. Lady Blackbird's character sheets have all the rules on them. World of Dungeons (you should go back the Dungeon World kickstarter if you haven't seen this yet) has slightly longer rules, but you can still read them in less than 10 minutes.
Short rules might require someone (the GM or host I guess) to bring a bit more experience to the table to get things going. But games that someone can play 15 minutes after having heard of the game for the first time are good things. I wrote a short game for Stage One last year, and then I wrote an even shorter game this spring.
Short rules require you to leave out all the rules that amount to "nobody be a dick at the table, we are friends and this is for fun". (Observe that, if playing pretend is the psychological [if not historical] genesis for the story gaming hobby, rules like stats and dice were made up in part to prevent people from being dicks.)
This post is already too long, so I'll stop. What are some good things with short rules? How can we squeeze big game ideas into small spaces?