When I teach Silverlight 2, I stress an important lesson that I thought that we (as developers) had learned the importance of linkability of the web. Early usage of Flash was the first time I noticed this. A number of those sites would create nested functionality that never changed the URL.  If the URL doesn't change, i can't bookmark it. Most Flash guys learned their lessons pretty quick, but now I am inundated with AJAX driven sites that try hard to not to do post-backs.  That's cool, but if the URL doesn't change I can't link to it.