For me it is o-line play. If that unit comes together and finds itself above average the running game will work and mediocre QB play will be a possibility based on decent protection. Mediocre QB play will, IMO, be enough to get to the playoffs based on the defense & ST.
Below average o-line play won't allow even mediocre QB play to become a reality.
Yes, after all this time I would call the playoffs a successful season.