Mecanim Character animation system, DirectX11 support, Linux Deployment... Nice!
While the main feature announcements sound very exciting, the little improvements will also make many developers happy.
Some questions might arise about the long-needed update to the GUI system. The current announcement states "Major optimizations in UnityGUI performance and memory usage". Will this also include any usability improvements? The current GUI system is not very intuitive to use and very limited. Let's hope that the GUI becomes as powerful and easy to use as the rest of Unity.
I'm very much looking forward to Unite 2012 in Amsterdam.
http://unity3d.com/unity4 (simplified html version)