I've just been through this, so it's fresh in my mind.
Dependency property system,
Routed Event system,
MVVM, Unit of Work, Repository and DI/IOC. MVVM is the main one, the others just appear frequently. There are a lot more frameworks around now than when I started in WinForms. Microsoft's Patterns and Practices site is worth looking at, Prism is pretty good.
Pro WPF in C# 2010 by Apress is my favourite book on the subject, others good ones include Illustrated WPF, WPF 4 unleashed and Sams teach yourself WPF in 24 hours. The last one is easy to get through in an afternoon and will cover the above in enough detail to get you started.
I created a diagram a while ago showing how most of the core WPF objects fit into a hierarchy. It's fairly interesting, I'll try and publish it somewhere and add a link here later.
Try and ensure the project you work on is using WPF 4. Visual Studio 2010 is much nicer, and WPF 4 includes a lot of substantial fixes that make it more reliable, faster and generally better.
Edit: I've just added the class hierarchy diagram I mentioned. It's good for an overview I think. Take a look and let me know if there's anything that looks wrong :)
WPF Class hierarchy