Some thoughts on this topic.
1. It's worth noting that the other team is allowed to play defense. Clemson was the #5 team nationally in defensive red zone TD percentage coming into the game. Even good teams -- OSU was the #3 team nationally in offensive red zone TD percentage -- struggle sometimes against quality competition.
2. North Carolina faced the #5 (Clemson), #9 (Pittsburgh), #11 (Miami), and #14 (South Carolina) defensive red zone TD percentage defenses this season. That was a quarter of the schedule. The Heels only scored 6 TDs in 14 red zone trips (43%) against those teams.
3. North Carolina had 55 total red zone possessions this season. The difference between where they finished the season (31 of 55, 56%) and the national median (61%) is only about 3 TDs.
4. Longo was only in his third season as an FBS OC last year. He's still learning on the job. Also, Mack Brown is an offensive coach and has a staff full of offensive quality control guys. Pointing the finger solely at Longo is lazy, although some of the play calling last season admittedly left me scratching my head.
5. Execution matters as much, if not more, than play calling. Case in point: on 3rd down and 5 against NC State, North Carolina ran a run-pass option (RPO). Howell could've handed off to Carter for what probably would've been a first down, maybe even a touchdown. He also could've passed to Brown, who had beaten his defender inside in zero coverage (no safeties back, meaning the end zone is wide open). Instead, Howell chose to hold the ball and take the sack. North Carolina settled for a field goal.
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There's every reason to hope that Howell, Longo, et al continue to improve with experience. No matter the reasons, it doesn't make sense that the Heels are likely to finish with a top-15 offense that's only top 40 in scoring. They have to find a way to turn yards into points.