I'm in the camp that expects the Cs to be comfortably better in both the regular season and the playoffs this season if everyone stays relatively healthy. Last season's team probably had about 65 wins +/- 5 games potential in the regular season...with bad luck they could have "only" won about 60 games, with good luck they could have probably won 70 or 71.
They return the vast majority of their regular season core, losing only 6th man Posey from the rotation and replacing his role with a healthier Tony Allen. Brown and Cassell had very little to do with the regular season 66 wins, and in fact incorporating Cassell late in the year probably led to a couple more losses than they otherwise would have had. I expect Rondo in particular, but also the 3 young bigs to all be markedly improved this year and to also take on a larger role in day-to-day energy production for the squad. And that's not even considering the comfort of everyone knowing their team role from day 1, instead of learning on the fly, which should also lead to team improvement.
While some believe that the team will be less focused (especially on the regular season) after winning a title, or that the competition has closed the gap, I think the exact opposite. If they stay healthy, I think this Celtics team has a very good chance to win 70 games next year. It's possible they could fall back in the regular season, but I'd be shocked if they only won 55 games barring catastrophe. Put it like this,I'd be more surprised to see them win only 55 games than I would be to see them break the Bulls' 72-win regular season record.