In simple terms, a responsive web design figures out what resolution of device it's being served on. Flexible grids then size correctly to fit the screen.
Smaller Footprint. Lightweight.
The new Bootstrap 3 promises to be a smaller build. The separate Bootstrap base and responsive.css files have now been merged into one. There is no more fixed grid, only fluid.
And lastly, this one. Flatness.
A big design trend for 2013 is "flat" design. Gone are the days of excessive gradients and shadows. Designers are producing cleaner flat designs, and Bootstrap 3 takes advantage of this minimalist trend.