From a pure feature point of view, this release might not be very impressive, but if you are interested in JavaScript and do a lot of custom development, this is the release you have been waiting for. One of the main focuses for the new 3.x branch is to produce a more powerful API and also make it output valid XHTML code by default.
One of the most notabled changes is:
Reduced the over all script size by 33% and the number of files/requests by 75% so it loads a lot faster.
… which I quite like 🙂