Bane is a Modern English word meaning "that which causes ruin or woe." Similar in meaning to the word problem. A Main Bane is the primary problem, something that screws up everything to the point where progress or success is never possible.
Closed-minded and undereducated people are the Main Bane to society, and are also more likely to be Raider fans.
JaMarcus Russell may not be able to hit the side of a house with a football, but any Raiders fan worth his salt knows that owner Al Davis is the Main Bane of the Raider Nation.