I was stationed there for three years and lived there for another 18 months as a civilian. Okinanwa is a wonderful place, the people are friendly and accepting. The main trouble they have with US forces is the live fire exercises done in the northern (rural) part of the island, and the Marine bases associated with that area.
There have been a number of assualts and rapes perpetrated by marines from Camp Hansen, which is an infantry base. Add to that trouble (although less so) from the air base and other marine bases, and that adds up to alot of ticked off Okinawans.
There economy will most certainly take a huge hit. Japanese workers guard the base, operate the base exchange and supermarket, run the officer, nco and enlisted clubs. The facilities are cared for by Japanese electricians, plumbers, pipe fitters etc... Many higher ranking service people bring there families with them to the posting, and live off base (not enough on base housing) renting apartments. Each unit has a "mama-san" who does laundry for them (and they are paid well). All this adds up to millions upon millions of wage dollars, not to mention the tourism, restaraunts and bars that are frequented.
Naha harbor does a booming trade with US Naval ships coming in and out of port, Naha international airport as well would be affected by the loss of business.
If that many people want us out of Okinawa, we must have really pissed them off.
edit: they are also pissed about the heavy flight traffic at the Futenma air base