Like jonasan13 said, it really depends on the game.
Here's my system for writing rules and what I've learned.
-Don't number them. Instead have it titled with a keyword or bold a keyword in it. It's easier to find and then your list doesn't look scary.
-Be inclusive. If you have seen it be a problem anywhere, make a rule. It's better to have a rule than to have someone later go "that's not in the rules that I have to play with more than just Draco in this community."
-Sort. It makes things seem less massive and a lot more approachable if you sort things into Icons, Players, Characters, Threads, etc.
-Make sure you address: the Ooc/Ic Line (powerplaying, adlibbing, but also taking IC conflict personally), Activity requirements (there is nothing that scares away a prospective player away more than no activity requirement), where you stand on canon, any specific posting formats you want, but also what belongs IC and what belongs in journals (owls, notices, emails, etc)
-If you're making rules about PBs, avoid banning people and instead make it a rule that PBs be age appropriate. If you ban people, make it a blanket ban (Anyone from the Harry Potter movies rather than a specific list of actors)
-Don't have the "we reserve the right to boot you at any time" rule. It used to be good to cover your ass on things you didn't think people were crazy/dramatastic enough to do. It's out of fashion, I suppose and it makes people think you're just going to start kicking people you don't like.
-Be clear. Take the time to find the right words. Example: "All characters are required to have an IC thread every fourteen days" means you could be hunting through every thread to make sure someone tagged. "All characters are required to ORIGINATE an in character action thread once every fourteen days" means they have to start the thread. It makes your players more proactive and it makes sweeps easier.
Whenever I want to make sure I didn't miss a rule, I look back at my old games. I would say look at games in ad communities if you aren't sure and see what rules you find reasonable and what you see as a turn-off.