The starting position for the lift makes the lifter susceptible to rounding the back, especially as you increase the weight. That's obviously not a good thing, since a rounded back while trying to lift something makes you prone to back injuries. You just want to keep your back straight throughout the lift. Also, keep your arms straight, too. You want to drive your feet into the ground to force the weight off the ground, not your arms. You obviously want a tight grip with your hands, but you aren't actively using your arms to get the bar off the ground. Think of them more as hooks. Give this a look, too, for a more detailed (and informed) rundown on improving your DL.