1) Special Teams - This unit was pathetic. If they weren't giving up a big play they were having one called back. Over the course of the year the special teams cost the Steelers at least one game, probably more.
2) In game decision making coaches and players - If Mike Tomlin makes better coaching decisions on game day, Ben makes better decisions with the ball at crucial moments, the running backs pick better lanes, the offensive line limits the brain cramps, the defense takes better angles and/or makes better calls the Steelers make the playoffs. In other words the details, a lot of little things added up to losses that shouldn't have been and typically this is the difference between 12-4 and 8-8.
Wholesale changes because of an 8-8 season are for the shortsighted and not typically how the Steelers operate (thankfully). Of course, the salary cap is going to force a change in the roster and it will be significant. So, the 3rd item in my list is identify the future, Lewis? Ta'Amu? Beachum? Whoever it may be and do whatever is necessary to get them into the fold, since the roster is going to change drastically.