Definitely mounted combat, as far as the base game is concerned. I came to Mount and Blade back in the day specifically for this reason, after seeing a short video on Youtube. No other game does it, and if they do, it's nowhere near as realistic or well done.
As far as cRPG goes, it's definitely the RP elements, and the camaraderie. Unlike Native, people that play cRPG play to progress, to build their character up, get cool new gear, so they keep coming back. In Native, everyone pretty much played where ever, you didn't really get a chance to get to know the players from one day to the next.