One of the most common questions I'm getting here at TechEd is, "What's new in WSE 3.0?"
The single biggest answer is that it is completely integrated with Whidbey (Visual Studio 2005 and the .NET Framework 2.0). You are required to have the .NET Framework 2.0 installed to install the technical preview of WSE 3.0.
Beyond the Whidbey integration are the things like the improved policy and security session support. It is also Microsoft's first implementation of the W3C standard MTOM spec for sending binary data with your XML message.