What's so hard about Event-Driven Programming?

I was lucky enough to attend the Software Architecture Workshop in Cortina recently. It was a three day workshop based around the idea of Open Spaces, which involves handing the asylum keys to the inmates and seeing what happens. I convened a session called “What’s so hard about Event-Driven Programming?” to explore the experiences of the other delegates in designing, implementing and testing asynchronous, event- or message-driven systems. I took the position that actually it was all very straightforward as long as you followed a few basic principles.