This looks like a nice set of changes for Bootstrap. I'm not sure how I feel about the grid class name changes, but then again, I went all-in on Foundation a few months back, so it doesn't affect me one way or another.
I'm sure mdo and fat have excellent reasons for using Less instead of Sass, but I'm a Rails user and I like Sass better to boot. Anyway, no matter, I'm glad to see Bootstrap is continuing to push forward!
http://monosnap.com/image/Vc05ZRQmGTmRrYvTEbLaYAI6w.png -- those do not look interactive at all.
[1] The trickiness with flat design is when buttons for action are also used as information display. One of the biggest issues with the first iteration of Metro was that you'd have a box that shows your current number of emails -- and you didn't know if that was just a part of your dashboard or also a link to your email that had a nice preview. That's not a failing of flat design, that's a failing of design.
Of course context plays a huge part, but it's a gamble whether this trend will survive for long.
Replacing all the markup has been on hold pending BS3 (the mobile-first aspect of BS3 seems a big win over BS2), but while waiting I've seen a lot of people talking about Zurb Foundation and its "superior" grid system.
From a purely HTML5+CSS3 point of view, what do you think of BS vs. Zurb?
That being said, my current setup is Stylus+Nib. (And for the above examples, you'd just write "border-radius: 5px" and it takes care of everything. Much more flexible.)
I think mixin's are generally overused; having to maintain css as well as right it, I'm quite happy with a syntax that doesn't encourage their use. If you need em: fine, but the you won't care about @include vs. "." anyhow.