If you only look at one feature in Windows Workflow, look at the Policy activity. The Policy activity processes rules. Business rules, game rules - any type of declarative knowledge. Read more in the latest OdeToCode article - "Windows Workflow - Rules and Conditions".
The Policy activity is easy to use, and provides a boatload of functionality out of the box. You can prioritize rules, and track rule processing in detail. WF provides an API to modify rules at runtime, which provides a great deal of flexibility. Rules execute with forward chaining semantics, meaning the rules engine analyzes the dependencies and side effects of each rule, and can reevaluate rules when the underlying data changes. I can see applications using Windows Workflow solely for the Policy activity.
More Workflow articles on OTC:
Hello, Workflow - an introduction.
Authoring Workflows - a look at XAML, XOML, and Workflow compilation.
The Base Activity Library - a tour of the out-of-the-box activities in WF.
Hosting Windows Workflow - a look at using the persistence, scheduling, tracking, and transaction services in WF.