If I had to pick just one factor, I'd go with Cora, even though I voted for regression, because I was expecting our offense to be much better this year even without the additions of Cora and JD. And someone had to vote for that option.
Of course, it's a combination of several factors.
1. Cora, as I have already mentioned.
2. Positive regression, which was not just possible, but highly expected. With that, Cora gets some credit too. He encouraged our hitters to be more aggressive, rather than taking that first pitch fastball right down the heart of the plate. Also, I heard a couple of days ago that part of Bogaerts' improvement was due to Cora telling him that he'd rather Bogaerts be a .285 hitter with some power than a .330 singles hitter, or something to that effect.
3. JD obviously lengthens the line up. In terms of being a positive role model and taking some pressure off of the other guys, he has helped as well. In terms of providing line up protection, no can do.