Ladies and gentlemen, the lower planes just became more dangerous.
That's a good thing.
Changes to the stat-block format - a good thing, having information at hand when neccessary.
Outsider type beefed up - That's a thing I'm concerned about. They were already rather powerful to begin with, and now they gain feats as characters, it appears.
Facing - No more odd facings I'm a bit sad about, but overall it could prove to be a good thing. I hope they don't adopt the standardized facings of d20 Modern - Some creatures are a bit bigger than that!
New DR - Overall, I believe this is a good change. Most characters will still be able to hurt beasts once in a while, even with magical weapons. Plus, the seemingly expected/feared flood of special materials might not happen. According to the compiled info:
That's right to bypass the Pit Fiend's DR15/holysilver, a Paladin will need at a minimum a +1 holy silvered weapon
Holysilver seems to be a magical silver weapon with the holy enhancement, not a single material. (and I would expect a few sentences like "This material x functions as material y for purposes of DR).
New ability scores for pit fiend - as Darkness said, these stats make some deities look pale in comparison.
New subtypes - Yes! YES! YE ... er... sorry
Tactics are a good addition.
(And I don't think this is CR 16 anymore.)