ORIGINAL: Blackhorse
Esprit de Corps: The Marines started the war as a small force. The "Old Breed" (pre-war Marines) numbered two understrength divisions. The Corps never had more than 6 divisions, and elements of those were created by canabalizing other elite non-divisional Marine units; such as ranger and parachute battalions. The wartime expansion - 2 divisions to 6 - was proportionally small enough, especially compared to the US Army, that there were always enough seasoned veterans to set the tone for a new unit.
Probably the most important factor. The US Marines FELT the were special..., a "cut above" as it were---so they tended to fight that way.
Napoleon found the same "trick" worked with his "Young Guard". Basically just conscripts, but given a fancier uniform and distinctions, they performed better than other conscripts trying to live up to the "Guard" tradition. Marine training and indoctrination has always emphasized that Marines were different and better than everyone else, and the average "jarhead" has strived to live up to the "rep".