This is an exciting time to be a Bootstrap developer. Finally, a css layout framework that makes sense! With the new release of Bootstrap 3 there are a number of important and timely changes in the works.
First and foremost is the new "Mobile First!" approach, which means the core principle of the Bootstrap grid has been adjusted. There are no longer separate files for bootstrap and bootstrap-responsive styles, instead the responsive grid is baked into the core.
A few other notes from my first foray into BS3:
_ variable names have moved from camelCase to hyphenated-lower-case syntax. This means some good ol' find and replace exercises if you're planning to upgrade a previous bootstrap project
_ the grid system no longer uses 'span' classes, bringing in 'col' instead. This means more f&r within your markup, and perhaps an overall change to the way your pages are set up, as there are also new patterns for definining columns based on device size. Good examples here: http://getbootstrap.com/css/
_ there are not very many Bootstrap 3 templates available yet (as of this writing, BS3 is still in Release Candidate stage). While there are still bugs being worked out and some adjustments being made by the dev team, we can expect the file structure to stay the same. This would be a great opportunity to create new Bootstrap 3 templates.
_ the documentation has been moved outside the project, which makes sense and keeps things cleaner for developers.
_ the basic download includes minified css and jss files, while the full source code includes directories "assets", "less" and "js", dropping the former "images" folder with the glyphicons .png files.
See the comments in this pull request at https://github.com/twbs/bootstrap/pull/6342 for a walk-through of all that has been added or changed in Bootstrap 3.
I have a few new projects in the works using Mura CMS , which has a very nice starter template based on Boostrap 2. I'm currently exploring my options for updating the default Mura bundle, or diving right into Bootstrap 3 using basic BS example pages.
This article http://bootply.com/bootstrap-3-migration-guide has a good explanation and comprehensive lists of find and replace commands for bringing a BS2 site into BS3. Considering all of the changes involved, upgrading Bootstrap 2 sites may become a source for new projects!