The Bootstrap 3 grid is fluid only. This example shows how to use a custom container to create a fixed width layout.
Responsive by default, and a clean "flat" design.
3 responsive grid sizes to control the grid layout on tiny, small and large displays.
New panels, list groups, glyphicons and justified navigation.
The optimized CSS is in a single file. Glyphicons are now separate.
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.