I have some information that I haven't heard mentioned before that Mountain Lion will have a new feature called Auto Quit that is different than Auto Termination in Lion. In Lion when you close the last window of certain apps (e.g., Preview) and switch to a different application, Preview will quit itself. In Mountain Lion, certain applications, when left idle, will quit even when the user does not close the window. However, instead of the application showing up as closed, a screen shot of the last view of that application will remain with a spinning gear overlaid on top of it. When the user selects the window again, the application will reopen in the background and the window will become active again. An example of an application with this behavior is the new Calendars application. I am not sure if this is built into the most recent developer builds of Mountain Lion or not, but I hadn't seen it mentioned from users of those builds.