We have made a number of changes and improvements that will be of interest to developers:

• We have upped the minimum requirement for Qt4 to version 4.8. Qt4 is one of the main C++ toolkits / libraries we use to create QGIS.
• New QgsStatisticalSummary class for calculating statistics from a list of numbers.
• Allow changing vector layer data source.
• Implicit sharing of classes : QgsField, QgsFields, QgsDataDefined, QgsFeature, QgsGeometry.
• Plugins can now create their own entries in the browser widget with two new classes : QgsDataItemProvider and QgsDataItemProviderRegistry.