http://www.wowinsider.com/2007/08/03...w-class-panel/

The above link describes hero classes as:

"What are Heroes?
Classic Warcraft 3 characters. Either didn't fill enough of a primary role in the original WoW or didn't feel right being played from level 1.
Epic feel -- should be very different from the other classes, such as the new resource system for the Death Knight."