This new version doesn't contain any major visual changes or addons but lots of modifications have been made behind the scenes.
The new cleanup engine that was previously added as a plugin has been moved to the core of TinyMCE this means that the old cleanup logic has been compleatly removed due to it's inflexible and old code and poor performance. The new cleanup routine is a lots faster and gives some new and interessting features.
The whole code structure has been modified, TinyMCE core is now no longer only one file it has been divided into numerous class files. These are located in a separate development zip/tgz file, this archive also includes all the needed development tools for generating a tiny_mce.js or tiny_mce_src.js file.
All themes and plugins are rewritten to a new interface OOP design. So they are now singleton classes instead of namespaced functions. Old plugins should still work since they get wrapped in a object when they are loaded.
Some of the TinyMCE core logic has been moved from the TinyMCE class to new separate classes like TinyMCE_Selection and TinyMCE_UndoRedo this breaks compatiblity with old plugins and themes but it should be pretty easy to port them to the new code layout.
All functions in TinyMCE core has been documented in API documentation (jsdoc) so it should be easier to understand how the internals of TinyMCE works. The documentation on how to write custom themes and plugins has also been updated, so it should be easier to create custom plugins.
Some old function and callbacks has been removed, this might break compatiblity but there are far better alternatives to this logic.
Lots of small bugfixes where also added to this release, and the overall performance has been looked over. The script contains more logic but it's now smaller than the previous release.
The cleanup of the code base and the refactoring was needed since some old functions have been in there for 2 years now and simply didn't performce well enough anymore. And it was harder and harder to maintain a file of 6000 lines of code so it needed to be separated. :)