Which position group will you be watching the closest this year (with either anticipation, dread, or both)? And which position group do you believe is the key to our success this year (defining success as 7-5 or above)?
QB would be the easy choice, and it may be true as well. D-line has depth issues, DB's were poor and gave up huge gains, and it's fair I think to say that the LB's were meh. The O-line was lost, especially early season, before improving late, and the WR's likely were not given enough chance to be difference-makers given woeful QB play. There is reason to believe each group should improve, some markedly, barring injury.
With the disclaimer that obviously we need all groups to step up, I will say that D-line pressure and stoutness against the run is the key*. Given a new scheme, freshman likely to play in rotation in all defensive position groups (plus Stevie Brown at a new position), and what is likely (at the beginning of the season) to be a precarious level of confidence given last season, that's my choice.
*I could make perhaps a stronger argument that O-line play is key, but assumed that would be the majority view, so there you go.