The main changes in this version are to the theming system. There are also a number of enhancements to the theming system that make it easier to modify themes and we’ve renamed a number of functions to have more informative names. Your existing code should continue to work, although you may receive warnings about functions that have been deprecated. Replacing them with new versions is easy. Here are the changes you are likely to encounter:

Other improvements include the addition of stat_ecdf, defaulting to the colour bar legend for continuous colour scales, nicer default breaks, better documentation and much more (including many bug fixes). You can read the complete list of changes on the development site