I think the difference is that everyone (me included) over reacted to the bad year. In Sept. 2011, they were in first place and were the hottest team in baseball. That somehow carried over and Lackey, Lester, Buchholtz and most of the other pitchers all had career bad years in 2012.
The truth it turns out is that the Sox pitching staff is closer to the 2011 version (June 1 - Aug 31) than the 2012 version. The core of the pitching staff is the same (Lester, Buchholtz, Doubront, Lackey) and many of the key position players are still here (Pedroia, Ortiz, Ellsbury, Salti, Nava).
With the Celtics, their heart and soul is gone (Pierce and KG plus to some degree Ray Allen). If the Celtics are by some miracle good this season, it will be due to new players, not a returning core.
And I realize the Rondo, Green, Bradley and some others are returning but this just isn't the same as getting essentially your starting rotation back and playing well.