The only thing I can think of was that the team only got up to play teams on the road, with the mentality of "let's embarrass them on their floor", and couldn't get up for the home games. Given the talent and level of play shown in the playoffs, I think it's safe to say they weren't playing up to potential and were playing selectively all year.
KG was also coming back from an injury and he kept improving all year. He went from getting blown by embarrassingly by Al Harrington to being very good and pretty explosive and quick in the Playoffs, dominating matchups including Gasol (at times, though especially on offense once he figured to face-up on the post up and blow by him), Jamison, and Rashard.
Rondo may be the MVP of the team but KG still provides the heart and soul, especially of the defense.
This year we'll have a better KG and with the new additions and younger legs playing more minutes (with regards to Nate, Shaq, Wafer, and whoever we get) I think we'll be better at home. But I still expect a good deal of the "coast" mentality to the regular season we saw last year. But we should have won the title, so that's not a big deal at all obviously.
Playing your hardest at an old age is risky in creating injuries... I'd say this Celtics team would have won 3 years in a row if they had managed to be without injuries. And anyways, I would rather win the ring than 10 more regular season games. There are a lot of good teams in the East now so I don't know how important the seeind really is.