The one thing I see most neglected in teaching a roundoff is the entry -- that is, the hurdle and lunge.
Different coaches have different preferences regarding how high or long a hurdle should go, but there are two key things that I think most coaches will agree on:
1. The hurdle should push forward; it should not go for height at the expense of forward momentum
2. It should land in a long, low, deep lunge. The lunge is almost always overlooked, but that is the primary source of a tumbler's power. The front knee should bend deeply, so that the gymnast can start lower and get her hands to the floor quicker. This also allows the gymnast to get a better push off the front leg, and a better drive with the back leg.