A release candidate of Bootstrap 3 was previewed earlier today. The last major release dates back to Jan 28th 2012 ... more than a year ago.

Personally, one thing struck me : the flat design. But in a very positive way, mainly because of two things :

I love super-minimalistic UIs.
I can't stand prototyping my apps with a Twitteresque design (I have a Bootstrap flat fork for that).

The bad news (at least for me) is the flat design is only temporary.

The other bad news, except the default responsiveness (responsive styles are now included in the main stylesheets - navbar example), I can't see any major change in Bootstrap 3. At least for the time being !

Bootstrap 3 - Responsive Layout

Bootstrap 3 - Home

Bootstrap 3 - Mobile Navigation

Get it now.

If you want to get while hot, and can't stand the teaser download link, here's a direct link via the Bootstrap 3 GitHub working branch.