Here are some of the highlights:
- Added a handful of new Less variables for easier customization.
- Removed recent progress bar changes for low percentages.
- Removed all instances of translate3d as they improved repaint performance, but also added several cross browser bugs.
- Added transforms to improve carousel performance in modern browsers.
- Updated Normalize.css and our H5BP print styles to their latest releases.
- Improved accessibility for navs, panels, tooltips, buttons, and more.
Brief mention of Bootstrap 4 too.
- Updated grid system with at least one additional tier for handheld devices.
- A brand new component to replace panels, thumbnails, and wells.
- A completely new, simpler navbar.
- Switch all pixel values over to rems and ems for easier and better type and component sizing.
- Dropped support for IE8.
- Tons of form updates, including custom form controls.
- New component animations and transitions for several components.
- Brand new documentation (written in Markdown, too!).
- A new approach to configuring global theming options.
- And hundreds more changes across the board.
My apologies. I copy pasted it straight from Bootstraps blog. Didn't even look at the formatting really until I'd posted.