IMO, the only question marks defensively, provided we have good health, are Hanley and Xander. Pedroia and Naps are Gold Glovers - not much improvement needed there.
Our defense at 3B last year was terrible, to the sum of -18 DRS. Sandoval playing decent 3B will be an improvement. He should be slightly above average.
Bogaerts improvement remains to be seen, but I am guessing that he will be improved over last year.
Small samples and all, but from what I've read and seen of Castillo and Betts, they are very capable of playing solid outfield defense. Hanley will probably not be that great, but I am inspired by his willingness and desire to learn his new position.
Overall, I think the defense will be very solid. Then again, I like Kool-Aid.