There are some sneaky teams in the east to contend with outside of Biston this season. Of course, the Bucks, Raptors, and Heat are there at the top, but to get to the finals there aren't going to be any "pushovers" anymore. The Nets, Hornets, and Sixers are still going to be competitive matchups with potential for upsets in a playoff series. The Pacers and Hawks could be a pesky eighth seed as well.
Ultimately, the Celtics are well-positioned to represent the East in an NBA Finals this upcoming season. But to @Troy Brown's point, expectations of such a thing should be met with the reality that they are still more than likely counting on one of their rookies or second year players to be a key contributor off the bench, (on offense in particular).
Nesmith can shoot at an NBA level and Romeo Langford can handle/defend at an NBA level too already. It remains to be seen if these young guys are 1.) going to remain healthy and 2.) rise to the occassion when it seems like it is inevitably going to fall on them.
The biggest improvements to the Celtics roster thus far seem to be how the players flow between rotations with their skillsets. The several years Ainge has constructed a roster with some players that didn't really fit into the rotation well. Now it appears the C's will be much better between first and second units.