The Close Window button appears to only send a request to the application to close; if the application is not responsive, it waits for a while before presenting a force close dialog. I've also seen at least one case of an application apparently accepting the request but not handling it; as a result, I have to manually kill the app. I'd like a button available to force close a window (and kill the underlying application if it's local) immediately. This does not need to be on the titlebar by default, as obviously it's dangerous and risks data loss, but it would be useful to have it available. If the application is that badly hung, I probably do want to kill it immediately rather than risk further loss.
You can kill applications with one click by using the "kill window" shortcut. By default it's bound to Meta+Ctrl+Esc. I assume that's quick enough?
It's much less obvious than a window button.
But as you said you don't want it to be on by default, at which point we're not solving anything
It's easier to find in the window menu or the decorations settings than an obscure key binding.