This is an opinion post. I have little stats that back this up.
Basically, I'd suggest that the reason the Celtics were not fun last year, and the reason for the young Cs lack of development, had to do with rhythm.
The best NBA teams have a certain chaotic flow and rhythm that is difficult to defend. The best NBA players facilitate that chaos. Steph Curry is probably the best in the NBA at this today, but it's what made Magic and Bird so special years ago. When they are on the court, teammates synced up with them. The offensive sets were not a grind. They flowed smoothly, not rigidly.
Last season, the Celtics didn't have that, like, at all. That's why they weren't fun to watch. They grinded (not a word, but you get what I mean) through their sets. Irving methodically picked apart defenses, but with very little team rhythm. I never had a feel as a fan for what Irving was going to do, which effects the anticipation of the crowd and eruption when success happens. Horford was methodical too. The offense ran through Horford and Irving effectively, but without chaotic energy, creative rhythm, or excitement.
This is one of the reasons the Cs were so much better a few years ago with Rozier as point guard. Rozier was not a better point guard per se. But Rozier does have a chaotic flow to his game. His energy and aggression creates the chaos. CBS has given him rhythm. (This is one reason why I'm not as down on Rozier as others. There's a chance he breaths youth, tempo, chaos, and excitement into Bridges, Monk, Washington, MKG, and Zeller)
If you want, you could go back further. That chaotic rhythm was what made the Cs so special with Thomas, Crowder, and Bradley. They synced up. They ran sets, but they also dipped into chaotic creativity. The team well out-played what everyone thought would happen.
I think this is why Brown and Tatum did not take a step forward last year. Both guys can run sets and score effectively in those sets, but both guys would work better (and have worked better) in a chaotic rhythm (most NBA players would). Brown especially, but Tatum too. If there is no rhythm to the offense, no juice, no dash of chaos, then Brown and Tatum will get points relatively efficiently, but not at the level you want for budding young stars.
For example the famous 19-0 by the Celtics against the Warriors started almost exactly when Irving left the game. Smart, Rozier, and Brown led a chaotic rhythm on offense and defense. The team and fans were locked in. Everyone felt that chaotic rhythm, and it led to a fun outcome.
https://www.youtube.com/watch?v=rdEycexAGvcI don't know if this makes sense, but this is one reason I think we will be better this year. I think Kemba and Kanter bring a bit of chaotic rhythm to the Cs offense this year. Both will play with more energy and tempo than Horford. Both will play with more team rhythm than Kyrie.
I think Brown and Tatum both blossom this year, but it will be in a bit of chaos, instead of the grind of Horford's and Irving's offense.
Getting excited for this team.