Defining all 4 screen sizes gets tedious. That aside, The new landing page on http://getbootstrap.com/ looks great
I believe that box is meant to be an ad, which the creators of Bootstrap use to support the project.
Perhaps the CarbonAds thing is just flaky
I've got to disagree, I think just having to add col size classes to divs to change how it's laid out on mobile/tablets/large monitors has helped tremendously, with 2.x it felt like we just had to hope it all worked out.