What's good about bootstrap: better documentation than Foundation, decent approach for the grid system. What's bad: too many styles you hardly ever need. Ok for proof of concept, not so much for most projects; No equal heights in their JS - foundation has this and it helps Poor attitude towards requests and comments from "users". Frankly, if I have a say in it, I just use the grid system and not even look the rest of it up.

Well, they didn't switch to flexbox, you can simply enable it instead of their normal grid system.

I think it's great they finally got to sass, that they've dropped ie8, and that they support flexbox. Seems like little a thing, but it is a necessary step and I'm glad they took it. Don't think there's a whole lot more to it, though