Here are the propably most important changes in the new version:
- Java 7 support
Language features of Java 7 are now supported. It is also default source level but can be configured via flag
- Debugging improvements
There are several improvements for Super Dev Mode, logging and error messages, because DevMode support in Chrome will be dropped sometime in 2014.
- Browser permutations
By default, the permutations for IE6/7 and Opera are disabled in version 2.6, and IE6/7 will no longer be supported in version 3.0. IE10 has its own permutation now.
Marker annotation to ignore classes, methods or fields during GWT compilation. So non-GWT-compatible Java code can be used in shared objects.
There are also several API enhancements (by adding “a more maintainable DOM event dispatch mechanism” and other useful methods), some cleanups (by removing deprecations), and of course a lot of bug fixes included.
The full release notes can be found on the GWT project page.