The better question is "what's the biggest area for improvement other than QB play?" QB play is obviously #1 right now.
The run game is right up there IMO. It was not great last season. We had a couple of good games (e.g. Miss State) and then Eric Gray had a couple of monster games at the end (Vandy, Indiana), but we nevertheless had one of the weakest rushing attacks overall in the SEC. Gray has to continue to play like he did at the end of last season, the O-line needs to improve on run blocking, and we probably need a couple more RBs to step up their game.
Overall, we need the run game to take a major step forward this year, particularly given that we're losing Jauan Jennings and Marquez Calloway in the receiving game.