ground vs load.
In a traditional deadlift, the loading is out infront of your feet. This creates a torque(shear force) that your body needs to balance. This is accomplished by many muscles, but is most felt in the lower back erectors. Since you need to stay behind the bar, your quads/anterior chain are not stressed nearly to the degree that your posterior chain/glute/ham/back is stressed. A great exercise for strengthening the posterior chain.
Trap bars have the load inline with your feet. This neutralizes the torque created by the bar being infront of you in a traditional dead(reduced shear force). Now the quads/anterior chain is able to assist to a much greater extent. This usually translates into a stronger lift. The stress has been moved forward. A great exercise for overall development.
Moving the bar behind your feet (hack squat) brings the force, as you would expect, towards the frontside.
Which exercise is better, is dependent on the individual. Having said that, If I had a trap bar at my gym, I would use it almost exclusively for deads with my athletes, especially heavy pulls. The reasoning being since they are doing olympic lifting, pulling from the floor all ready. They are getting enough of the traditional dead from the olympic lifting, taking some stress off their back, for the heavy pulls with the trap bar.