Tamara points us to David Amerland's Broken Windows post.
It's an oft-cited (though somewhat unverified) theory that if you clean up the minor misdemeanors, the big stuff goes away.
For online communities, the logic is like this. If you crack down on the little things you can keep the scary trolls away. So correct the typos, remove the off-topic posts, clamp down on the slightest variation from rules.
That might be true, but there is no data to support it (and a little that's skeptical).
The problem is this only removes the bad stuff. It doesn't encourage the good stuff. It might reduce the level of activity, upset members (we like flexibility in our online environments) and prevent the community from pivoting in a direction it wants to go. This is the problem with top-down planning.
What we do know from studies of social norms in communities is people generally do what they see other people doing. This is true in both online and offline environments. If you want to encourage positive behaviour, you need to worry less about the negative stuff and more about using your subtle influence to showcase behaviour you want to encourage.
This is reflected in not only what discussions become sticky threads, whose behaviour you draw attention to, what content you write about, but also in the little things. The personality of the community, the tone of voice you use, the first people you approach to join the community (this is a big one).
Discouraging the bad behaviour is an exhausting effort that wont get you far. It's smarter to encourage the behaviour you want to see.