kind of the million dollar question, isn't it?

I don't think there's an easy answer. I think the fact that eventually all communities homogenize and die gives most community planners ample reason to study the issue. That said, I think if you look at the differences between a geographic community and an internet community, the biggest difference is friction.

If your community is "Sedona" you had to move there. You had to sign up for electricity, phone, television, internet, garbage service. You found the restaurants, you found your favorite parks, you found the speedtraps.

If your community is "Reddit"....

If your community is Sedona, your neighbors might annoy you but you shop at the same grocery store, you stop at the same stop signs, you vote in the same elections, the same rain falls on you all. Loudmouths are bitched about around the dinner table, not the commons. Everyone just wants to get along and enjoy the community. You're all invested in it, economically, socially, emotionally.

If your community is Reddit...

This is part of what the points systems endemic to internet communities attempt to curb. They are "investment." However, they're not the same "investment" as "I can't move because I really like the Thai food down the street." What you need is something "sticky" to cause people to do things they don't want to do in places they don't want to leave. You need things to be difficult to set up and difficult to break down (ever tried to leave Facebook?) You need a social structure that causes people who don't agree and won't agree to somehow have to agree in order to participate in the commons.

The problem most internet communities face is their emphasis on fluidity, on ease, on transactional utility. Most real communities emphasize those things that cannot leave their community (parks, shops, restaurants, scenery). You're "stuck there" and have to deal with people's bullshit. Of course, it's generally something other than "people's bullshit" that attracted you in the first place.

With online communities, it's all about the bullshit. And since it's so easy to come and go, the only people left after a while are the bullshitters. The trick, then, is to add some sort of inherent value that emphasizes transactional friction - aka politeness, opposing viewpoints, etc.

And I have no idea how to accomplish that.