Carbon can be made in a far wider variety of temperments than steel or aluminum. The most comfortable bike I've ridden was full carbon, the least comfortable jumpy nervous feel-every-bump bike I rode was also full carbon (a scott).

My personal experience has definitely been that bikes with carbon forks and aluminum frames have been a much less buzzy ride than the full aluminum bikes I rode before them. (My full aluminum bike was a cannondale touring bike so not a stiff racing machine). 'course could be just better bike design in newer bikes as well.