Apache OpenOffice 4.0 has a new sidebar user-interface, more than 500 bug fixes, improved Microsoft Office interoperability, drawing/graphics enhancements, performance improvements, and a new logo.
The new sidebar UI that's dubbed as a major feature of Apache OpenOffice 4 was contributed by IBM from their Lotus Symphony software. This sidebar is also what's going to be added for LibreOffice 4.1.
The Microsoft Office interoperability improvements include support for MS Word 2003 Graphic Bullets, bookmark reference to normal numbering support, and numbering/bullet attributes determined by the paragraph end mark in Microsoft Word, among other small changes.
More details on all of the OpenOffice 4.0 changes can be found via the Wiki release notes at Apache.org.