1) Improved Performance - WF 4.0 is re written from the scratch, the new runtime in WF 4.0 promises 10 to 100 times performance improvements. Speed improvements are due to increase in execution speed as well as the reduced size of persistence information. To understand the what went behind to cause this improvement, refer this
2) New activities – Several new OoB activities introduced. Newly introduced Parallel activity helps in writing logic in parallel without needing to write explicit thread synchronization. State Machine workflows in WF 3.5 are no more around; Flowchart is introduced as a new workflow type. To understand list of various activities supported through WF 4.0, refer this
3) Fully declarative workflows and activities
Declarative workflows have been possible earlier but to build the UI declaratively using XAML in the business layer was not easy. In WF 4.0, it is very simple to create a workflow declaratively in XAML. Writing workflows in XAML eases versioning, deployment and maintenance. The advantage is there is no code (behind) and the workflow can be changed dynamically and deployed easily.
4) Better integration with WCF 4.0 (Workflow as a service)
Workflow can be exposed as service without needing to write WCF service wrapper.
5) Workflow persistence – State Machine workflows are no more supported in WF 4.0. The workflow persistence can be achieved using Persist activity and can be applied to Sequential or Flowchart workflows.
6) Correlation support – Correlation is the ability to tie up related workflow services messages to the same instance of workflow. Workflow 4.0 supports 3 types of correlations: context, content and message based correlation. Refer this to understand various correlation mechanisms supported in WF 4.0
7) Compatibility with the previous versions
In WF 4.0, System.Activities.* assemblies hold everything related to WF. But the older System.Windows.* assemblies have been retained to ensure backward compatibility, so your WF 3.5 based workflows will work in 4.0 through newly introduced interop activity. Check this for WF 3.5 to WF 4.0 migration guidance.
8) Workflow host (App Fabric) – Appfabric is used to host workflows and services. It provides out of box monitoring, scaling (instance management), persistence which otherwise will have to be implemented and managed through custom extensions.
9) Simplified Designer - In WF 4.0, the designer comes with performance and usability improvements. The workflow designer is completely rebuilt using WPF and this gives it a great, rich user interface. It is possible to create and open large workflows without loss in performance.
10) Re-hosting designer - In the previous versions, developers would need to write a few hundred lines of code to re-host the designer where as with WF 4.0 it is just a few lines of code. One can re-host workflow designer in WPF with a few lines of code.
11) Improved developer productivity – it takes 80% less code to write custom activities as against earlier version. WF 4.0 also removes the notion of a root activity; any activity can now comprise a workflow as compared to just Sequence and State Machine Activity previously.
While this is not an exhaustive list on what WF 4.0 has on offer. Am sure many would be keen to adopt and exploit what WF 4.0 has on offer.