LibreOffice 4.0, a milestone in the evolution of the Document Foundation's open source productivity suite, has been released with improvements that will be welcome by both users and developers. 

Announcing the new release Italo Vignoli, on The Document Foundation Blog, writes:

LibreOffice 4.0 is the first release that reflects the objectives set by the community ... : a cleaner and leaner code base, an improved set of features, better interoperability, and a more diverse and inclusive ecosystem.

Vignoli also notes that in the 30 months since it was first announced in September 2010, LibreOffice has become the largest independent free software project focused on end user desktop productivity and has attracted more than 500 developers, three quarters of them independent volunteers.

As far as end users are concerned there are significant improvements with one that is immediately noticeable being:

• Significant performance improvements when loading and saving many types of documents, with particular improvements for large ODS and XLSX spreadsheets and RTF files.

The other new features can be summed up as having something that will appeal to every user:

• Integration with several content and document management systems – including Alfresco, IBM FileNet P8, Microsoft Sharepoint 2010, Nuxeo, OpenText, SAP NetWeaver Cloud Service and others – through the CMIS standard.
• Better interoperability with DOCX and RTF documents, thanks to several new features and improvements like the possibility of importing ink annotations and attaching comments to text ranges.
• Possibility to import Microsoft Publisher documents, and further improvement of Visio import filters with the addition of 2013 version (just announced).
• Additional UI incremental improvements, including Unity integration and support of Firefox Themes (Personas) to give LibreOffice a personalized look.
• Introduction of the widget layout technique for dialog windows, which makes it easier to translate, resize and hide UI elements, reduces code complexity, and lays a foundation for a much improved user interface.
• Different header and footer on the first page of a Writer document, without the need of a separate page style.
• Several performance improvements to Calc, plus new features such as export of charts as images (JPG and PNG) and new spreadsheet functions as defined in ODF OpenFormula.
• First release of Impress Remote Control App for Android, supported only on some Linux distributions. (The second release, coming soon, will be supported on all platforms: Windows, MacOS X and all Linux distros and binaries.)

For its developers one feature that was already implemented before the 4.0 branch was:

• Improved code contribution thanks to Gerrit: a web based code review system, facilitating the task for projects using Git version control system (although this is not specific of LibreOffice 4.0, it has entered the production stage just before the 4.0 branch).

API changes of interest to developers include the bundled Python being upgraded from 2.6 to 3.3 which means that some Python extensions and macros may require re-working. The Windows/C++ compiler has been upgraded to Visual Studio 2010.

With regard to the "under the hood" changes Vignoli comments:

Overall excellent backwards compatibility is retained for legacy extensions, but moving forward TDF is committed to a more pro-active approach to evolving the UNO APIs, with more functionality to be deprecated, and eventually dropped, in due time – according to the six month release cycle – throughout the LibreOffice 4.x release series.

He also notes how much has been achieved in getting to LibreOffice 4:

During the last seven months, since the branch of LibreOffice 3.6 and during the entire development cycle of LibreOffice 4.0, developers have made over 10,000 commits. On average, one commit every 30 minutes, including weekends and the holiday season: a further testimonial of the incredible vitality of the project.

If you want to get involved in this ongoing open source project,  see the Development wiki page