There are a couple of problems:
1) They don't have a solid hardware platform to work on, and the feeling I've gotten from talking to some of the people on /r/darknetplan is that they really don't have the proper understanding of wireless tech to make something like this happen. People are proposing cheap commodity wifi gear for this.
While it's certainly possible to do a very high density area with that, it's certainly not feasible to cover an area like a city, at least not without a considerable investment, and an understanding that network performance is going to be abysmal. (And at that point, just use ham radio...)
(Something like what they're proposing might work in an office, not so much in a neighborhood).
So is a problem, but hypothetically you could just toss hardware at it until it went away.
2) It's the addressing, stupid. http://www.reddit.com/r/darknetplan/comments/m2nd5/its_the_a...
I am definitely the last person to discourage people from "doing it wrong" (http://thingist.com/t/item/4372/ - http://thingist.com/t/item/19766/ - http://thingist.com/t/item/21434/), but people running this, prepare yourselves for the reality that you could fail.