The Celtics front court rotation has improved. Lee replaces Bass, Johnson probably pushes Zeller down the chart a notch, and one of Sullinger/Olynyk is available for trade. Among Lee, Sullinger and Olynyk, Stevens can deploy big men with more legitimate offensive skills than in 2014-5.
The Celtics back court rotation has improved. There is the problem of having too many 1.5 guards, but if anybody can find a good offensive structure for their combined duplicative skills, Stevens can. All the important offensive components are returning in Thomas, Smart and Bradley. Rozier might provide a spark off the bench; the rest of the draftees are unlikely to be relevant.
The Celtics will have better personnel, through trades, signings, and maturation. Their competition will be better too, and with Lee and Johnson, injuries must be expected. Still, it's nice to have an incentive to win, and having Ainge and Stevens work with common incentives may help as well.
Nobody likes watching a tank race.