What is Shared State in an ASP.NET MVC Application?

Question

In a recent Stackoverflow podcast (#59), Jeff Atwood mentions that Stackoverflow has some "shared state." I understand the general idea of shared state, but what exactly is shared state in the context of an ASP.NET MVC application, like Stackoverflow?

Specifically, I'd like to know the following:

1. What does it look like? How do you spot it in the wilderness?
2. What limitations does it bring to a system?
3. How do you get rid of it?
4. If it's so bad, why would someone include it in their design?