Microsoft Workflow Foundation (WF) is designed to build a kind of Domain Specific Language. You can take WF as a higher level programming language. For example, it has "while" and "if" statements (activities). All the statements run sequentially.
At this time, WF is good for back end process, but not for (web) user interface. Why? Because WF does not support "Back" button. Let's suppose a web application using WF to control page flow. It is very common for user to input data across pages and want to go "back" to previous pages to change data. WF does not provide built-in mechanism to do that.
How to implement "Back" logic in WF? WF needs a stack to save states for previous activities. When user clicks "Back" button, WF should pop up previous state from stack and continue the "previous" work.
I hope the new version of ASP.NET integrated with WF can have that "Back" button feature.