When I think "community organizer," I think of that annoying girl I used to know who just a little bit too into her condo association.
I think of people who want the government to do something for them, either because it's already doing it for other people, or because there's a problem that needs to be addressed and institutions aren't doing anything about it. It can be about improving mass transit, providing senior housing, improving street lighting, getting traffic patterns fixed to ease traffic, reducing crime, any number of issues. It's bigger in the cities because you have more of a disconnect between people and their government and a lot of externalities that get foisted on communities with little power. You find this out when you are part of gentrifying a community that has massive air pollution from the interstate highway and 8 commuter rail lines running through the city and only one subway station within city limits, everything else requiring bus service.
