Full Width Layout
The Bootstrap 3 grid is fluid only. This example shows how to use a custom container to create a fixed width layout.
Mobile First
Responsive by default, and a clean "flat" design.
Powerful Grid
3 responsive grid sizes to control the grid layout on tiny, small and large displays.
New Elements
New panels, list groups, glyphicons and justified navigation.
Less Bloat
The optimized CSS is in a single file. Glyphicons are now separate.
Moving from Bootstrap 2.x to 3.0
Everything has changed, so you can't just replace the 2.x with the 3.0 files. The folks at Bootply, the Bootstrap Playground have created a migration guide to help.