Scott's blog has an extensive list of the new stuff: http://weblogs.asp.net/scottgu/archive/2008/05/12/visual-studio-2008-and-net-framework-3-5-service-pack-1-beta.aspx
If you scroll down about half way, you'll find the WPF features: better startup performance, target performance fixes in the other areas, a new hardware accelerated graphics effects architecture, DirectX interop and a series of data enhancements, most notably better support for writing data virtualizing controls. This is not your usual collection of bugs fixes...there is a *lot* of new stuff in the release and you'll want to check it out.
Looking forward to our next few releases, I think you'll see the same sort of priorities: constant perf improvements, better interop with other UI technologies, and more and better features and controls for data oriented applications. The core architecture seems to be complete and stable, we just need to flesh out the system and add targeted new features.