Brad Stevens figured out how a team should play D in this new NBA era of shooting and spacing. Everyone else prioritized offense, but Stevens knew better than that.
He is also absolutely aligned with Danny Ainge on what type of players to recruit to fit this system. They needed to size up their guards and wing players and they did just that. They needed agile 6'8" to 6'"10 forwards who can shoot from the outside and are mobile enough to switch through picks, and they got that too.
It's not a coincidence that teams dramatically get lower FG percentages against the Celtics and get less rebounds. We are looking at a legitimate NBA Championship contender.