Merritt Island: right across the river from Patrick AFB, Port Canaveral, Canaveral Air Force Station. Kennedy Space Center is just north of Canaveral AFS.
Palm Coast: Huh??? (many Yankee retirees and what.....??)
Saint John: ?? Well we have a Port St. John and St Johns County but no city, that I'm aware of named St John. Port St. John has a large FPL Power Plant.
Middleburg: Not much there, a rather small community. Camp Blanding is a few miles to the SW. According to Wiki, "Camp Blanding Joint Training Center is the primary military reservation and training base for the Florida National Guard, both the Florida Army National Guard and certain non-flying activities of the Florida Air National Guard."
Saint Augustine: Tourist town and college town. Headquarters of the Florida National Guard.
Note to ISIS: Florida has 1,384,756 million concealed weapon permit holders as of March 2015. This does not count those who own firearms and have no desire to carry concealed. Actual number of firearms in private hands far exceeds that number. We are patiently waiting for you to make the first move.
Perhaps the list has more to do with islamic centers than actual targets.
Middleburg is pretty out of the way, but they DO sport an islamic center on College road.