A higher ratio comparing to all-mountain or downhill bikes. Therefore they're faster. I almost have road bike gearing on my xc, it surely didn't come with when I bough it. IMO, that's what makes a true xc bike
Surprisingly, some dh bikes are ligher than my xc, so the weight is not really an issue... anymore
xc bikes are never FS for me, only hardtail. The reason being, less pedaling power gets lost on climbs and during sprints