That is strange. It works for me both with conventional (just checked FireFox 39, Opera 12, Visual Studio 2015, Notepad) and WinRT (Calculator) applications.
It turns out that I had all Settings or Control Panel type windows open when I tried that last time. Those windows don't have a new-window function.
So, I think if we need a reason to change the functionality of middle-click on a taskbar-item, that's it. Every window should have a close-window function and there would be no confusion over what middle-click means.
(Then again, I guess the designer was probably trying to protect non-power users when they decided that. A close-window function is obviously a destructive act. But power-users are such a powerful group, so I think they need to cater to use a bit more by giving us the gosh-darn options that we want!)
EDIT: Also, I take it back. I'll keep it on this machine for now and see what happens...
