WPF 4 - New/Enhanced features

WPF Tracing Support
http://10rem.net/blog/2010/04/12/wpf-4-release-a-guide-to-the-new-features

ClearType
http://research.microsoft.com/en-us/projects/cleartype/

Custom Dictionaries APIs
WPF Text Team Blog post on Custom Dictionaries in WPF 4

Pixel Shader 3 Support
Create Custom Pixel Shader Effects for
WPF http://windowsclient.net/learn/video.aspx?v=296117

Removal of legacy Bitmap Effects
The old legacy bitmap effects (bevel, drop shadow, blur etc.) are still present, but there's zero implementation. The effects that have shader versions (blur, drop shadow) get forwarded to the shader implementation. The other effects (bevel, for example) simply do nothing. Your code will compile, but the effects themselves are no-ops. In .NET 3.5sp1, they were marked as deprecated, so this is not surprising.
Bitmap effects are software-only, and couldn't be hardware accelerated. One of the biggest performance issues with old WPF applications was use/overuse of bitmap effects. Pixel shader-based effects are much more performant. I'm glad to see we aren't afraid to cull when we come up with something better.
The replacement effects are just as easy to use as the old effects, and will even render on the design surface.
http://10rem.net/blog/2010/04/12/wpf-4-release-a-guide-to-the-new-features

Microsoft Windows Presentation Framework (WPF)

The Windows Presentation Foundation, WPF, provides a unified framework for building applications and high-fidelity experiences in Windows Vista that blend application UI, documents, and media content. WPF offers developers 2D and 3D graphics support, hardware-accelerated effects, scalability to different form factors, interactive data visualization, and superior content readability.

Nice book on Microsoft WPF
Windows Presentation Foundation Unleashed (WPF)