On some XC bikes I notice that the frame is much like a road bike. Like my old mountian bike, before there were downhill bikes, it has a almost horizontal top tube with the ground like a road bike. But I love it for riding on the road or XC trails. I guess the other thing would be the tires it comes with (lower profile knobies) and the ~80mm shock.
But what else makes an XC bike an XC bike?