As Charles-H. Schulz, one of LibreOffice's founders and a member of its parent group, The Document Foundation, explained, "In a sense, the 4.0 is actually an existential release, as it marks the departure from the past, the inclusion of new technologies and a more coherent and effective story on licensing. … The 4.0 is not just an update, it represents a deep change for LibreOffice and enables us to come closer to fulfilling our mission: to create the tools for knowldedge and the instruments of freedom.
Schulz and The Document Foundation hopes that this API change "will allow, with time, for the introduction of deeper changes and a more powerful API. But it also means that, while the API is becoming more powerful and easier to tap into, new possibilities for extension developers will rise, with its set of changes and incompatibilities. On a more abstract level, these changes also mark a more radical departure from the OpenOffice.org codebase, and it is now becoming quite difficult to just assume that because OpenOffice.org, Apache OpenOffice behave in one specific way LibreOffice would do just the same. Of course the API changes do not make the whole work themselves, but the work we started with the 3.4 branch is paying off: LibreOffice 4.0 is becoming a different animal, and that comes with its own distinct advantages while clearly showing our ability as a community to innovate and move forward."
Besides tje API change, LibreOffice is moving to a new graphics stack. This is based on GtkBuilder, which builds interfaces from an XML user interface (UI) definition. The reasons for this change are to make it easier to create "new UI widgets, cleaner looks and new opportunities to handle new tools and improve our interface."
The Document Foundation is doing this for two reasons. First, it will make it easier to "incorporate any useful improvements" from Apache 2.0-licensed OpenOffice code into LibreOffice.
Second, they believe that the MPL licensing will provide "some advantages around attracting commercial vendors, distribution in both Apple and Microsoft app-stores, and as our Android and iPhone ports advance in tablets and mobile devices." In short, this is a move to help make future tablet versions of LibreOffice, due out in late 2013/early 2014 more compatible with Android, iOS and Windows Phone 8 app. store restrictions.
On Linux, however, LibreOffice will continue to be under the LPGLv3. "As the migration continues, and for the foreseeable future on free-software platforms we will continue to distribute our binaries under the LGPLv3 - in addition to the existing mix of external component licenses."
So, while this forthcoming version of LibreOffice may not have users racing to download it the second it arrives, developers should already start looking the LibreOffice release candidate, the new APIs, and the new licensing over.