There are few new features and tools such as better tools and documentation for animations, and new Visual studio 2008 templates. However the big news to my opinion is that SDK 5.3 made XBAP hosting deprecated. I tried to find why? And what does it mean? In the documentations, they delete anything regarding WPF (for example they now compare only two technology choices - HTML & MCML). In the template, there is no XBAP template. I wonder if old XBAP application still gets compiled with the new...