Per the Document Foundation release notes on LibreOffice 4.0 Beta 2, there's a whole lot of bug-fixing that went on for this second and final beta. Changes range from crash fixes to faster RTF loading to improved filtering, better presentation mode rendering, cell formatting enhancements, performance improvements, and other formatting enhancements to this suite of open-source productivity software.
LibreOffice 4.0 Beta 2 has advanced quite a bit since the original 4.0 alpha in November. Release Candidates of LibreOffice 4.0 are expected beginning in January while the official 4.0 release is expected by mid-February, per the release plan.
For those wanting to learn about all of the new features coming to LibreOffice 4.0, see the full release notes. This latest development release can be downloaded from the pre-release area.