I totally love mesh networking. I would put up with slower download and upload speeds to get rid of dependence on private or public entities to provide service that price gouge and stifle innovation. To those that say "it can't handle that much traffic", it can with smarter routing algorithms, and network usage is self-regulating (if users can't watch streaming netflix, they will stop trying). The net is about freedom, and mesh embodies that freedom.

The upcoming 1 Gbit 802.11ac standard should help mesh networks, too, in the coming years. There's also that "Super WiFi" thing (nothing to do with the current Wi-Fi standards) that is supposed to work over miles at once, but I assume the equipment for that will be very expensive and the nodes will be very centralized (owned by companies only).

Why isn't this capability built into every wireless router?

It is. Or do you mean why isn't it in the default firmware?

For anyone interested in how one might build such a thing, this PDF isn't a bad starting place: http://en.wikipedia.org/wiki/File:Building_a_Rural_Wireless_...

How do you have a "community mesh network" without encountering a tragedy of the commons?

By providing ample bandwidth for the users of your network, focusing on local services over remote Internet service, and not competing with traditional ISPs at serving up media and entertainment.