Top shelf defense is a good recipe for winning regular season games and making it into the playoffs. I think we've seen plenty of examples, even just in the last few teams, of teams that get there on the strength of defense and general competency, only to fold when it turns out -- surprise! -- the lack of of a dangerous offense means they can't cut it against talented opponents.
I think the Celts are in that boat, still.
We may also see in the next year or two that the defense will have to take a step back or two in order to integrate more talented offensive options (similar to what the Bucks are going through this year).
I agree with Nick that 45 or so wins is probably a better estimate for where the team will end up, given what we've seen so far. I wonder if the team won't regress to the mean just a little bit defensively, however. Top 5 in defense is a huge leap for this team after finishing last year outside the top 10. But maybe starting Jae Crowder full time and adding Amir Johnson to the rotation is making that big a difference? We'll see.
I don't buy the narrative that you can be a good regular season team with defense + depth but that offensive star power is needed to win in the playoffs. I haven't seen any statistical evidence to suggest anything of the sort, and my strong feeling is that winning is winning is winning, no matter what formula you use to do it.
It's possible that the C's defense will regress a bit over the course of the season - they're forcing turnovers at an outlier-y rate, and maybe teams will adapt to that and start taking care of the ball better. But on the other hand, it's a young team that progressed substantially over the course of last season, and maybe they'll progress this season, too.