Taken from 'The Dead Zone - The Top 10 Deadlifting Mistakes and How to Fix Them' by Dave Tate:
[i]Mistake #5: Not pulling the bar back
The deadlift is all about leverage and positioning. Visualize a teeter totter. What happens when the weight on one end is coming down? The other end goes up. So if your body is falling backward, what happens to the bar? It goes up! If your weight is falling forward the bar will want to stay down. So if you weigh 250 pounds and you can get your bodyweight to work for you, it would be much like taking 250 pounds off the bar. For many natural deadlifters this is a very instinctive action. For others it has to be trained.
Proper positioning is important here. If you're standing too close to the bar it'll have to come over the knee before you can pull back, thus going forward before it goes backward. If your shoulders are in front of the bar at the start of the pull, then the bar will want to go forward, not backward. If your back isn't arched the bar will also want to drift forward.
For some lifters, not being able to pull back can be a muscular thing. If you're like myself, I tend to end up with the weight on the front of my feet instead of my heels. This is a function of my quads trying to overpower the glutes and hamstrings, or the glutes and hamstrings not being able to finish the weight and shifting to the quads to complete the lift. What will happen many times is you'll begin shaking or miss the weight. To fix this problem you need to add in more glute ham raises, pull-throughs and reverse hypers.
Mistake #6: Keeping the shins too close to the bar
I'm not too sure where this started but I have a pretty good idea. Many times the taller, thinner lifters are the best pullers and they do start with the bar very close to their shins. But if you look at them from the sides they still have their shoulders behind the bar when they pull. This is just not possible to achieve with a thicker lifter.
If a thicker lifter with a large amount of body mass ? be it muscle or fat ? were to line the bar up with his shins, you'd see he would have an impossible time getting the shoulders behind the bar. Remember you need to pull the bar back toward you, not out and away from you. So what I believe happens is many lifters look to those who have great deadlifts to see how they pull, then try to do the same themselves. What they need to do is look to those who are built the same way they are and have great deadlifts and follow their lead.[/i]