If you want more information, you can read more about the framework here. They just recently changed from Version 2.3.2 on their website to the Release Candidate for Version 3. If you are starting a new website that focuses on mobile, and allows you to ignore IE7 then definitely use Bootstrap 3. To read more about the release check out their blog.
The other main thing that they removed from the Bootstrap core is their Glyphicons. Basically these Glyphicons are just small icons that you can easily embed pretty much wherever you want. Also, creating icons can be a difficult task, especially if you have no graphics skills. The glyphicons are available in a separate project found here though.
If you are ready to create responsive websites, and want to create them as easily as possible, be sure to check Bootstrap out. Leave comments with your thoughts!