Do you want to know what I think would be a good feature of OS X? It could give users the option to easily program OS X so that when it shuts down or reboots, it will force quit applications automatically if they fail to quit within a certain period of time.
Sort of what the VM on my Mac does - tells the app to close, after a very short while - if app is still there then asks me what I want to do. If app closes in the meantime while VM is waiting a response from me, the VM closes the dialog box as there is no longer any point in asking me and just gets on with shutting down. Smart.
What Apple does is - let me select shutdown, have me confirm it on a dialog box and let someone that wrote the Mail code decide no we are not going to do that.