Size kills. The Celts had 4 guys 6'10" and over. They would impose their will on the Warriors, who just wouldn't have the personnel to match up over 48 minutes.
I dunno, the Warriors' death lineup has been killing size for most of the last two years.
What size have they gone up against that is remotely equivalent to 6'10" Larry Bird / 6'10" Kevin McHale / 7'0" Robert Parish / 6'11" Bill Walton?
Thunder are pretty darn big with Durant, Ibaka, and Adams starting and Kanter and Collison on the bench. Now none of those guys have the interior scoring of McHale, but the size is there. Cavs aren't exactly small either when they are healthy with Mozgov, Love, and James starting and Frye and Thompson on the bench (I know they didn't have Frye when they played the Warriors either game this year, but they did have Varejao).
People forget that the Warriors have 4 guys legitimately 6'10" including Bogut who is an excellent defender (Ezeli, Varejao, and Speights are the others).
That said I would expect the Warriors to play their usual rotation with a bit more Bogut, which begs the question, who the heck are Bird and McHale going to guard? Neither has the footspeed to stay with Barnes or Iggy and I can't see McHale actually guarding Green on the perimeter. And while Ainge and DJ were solid defenders, no way they can stay with Curry.