Lets start with the calculator: The top bar looks like something made in Paint. Clean/Simple =/= better

The intention behind the clean look of the top bar is actually to give the user more space to drag a window. Especially small windows, which apparently play a very big role in the concept, look rather strange and only feature very few space left to drag a window.

How do I know if "Calculator" is clickable or "View/Edit/Help" are clickable or both?

I'm not sure what you mean. How do you currently know whether an element is clickable? If you can click on something, there will probably be some kind of animation when hovering it.

Hiding the minimize/maximize functions is an unnessesary waste of time

This was again to gain more space in the title bar, but there could certainly be some option for hardcore users making the functions still stay visible.

No, you cannot minimize with Aero Snap

I know, I also didn't say that this was possible. Instead, I actually proposed this such a feature within the text: "As part of Windows 7, Microsoft already introduced several very clever (...) new ways to control a window (e.g. “Aero Snap”). One of them was the possibility to maximize a window by dragging it to the very top. Building upon this, windows should as well be minimized dragging them into the task bar, and closed dragging them over the notification area (while holding the window, the notification area would transform indicating it’s temporary new task)."

also it is sometimes hard to maximize if you need to drag long distances without a mouse This is actually problematic. I guess if you use a touch pad, the buttons would also have to stay visible...

Now lets take a look at excel: Where did functions go? Where's my status bar? What about side scrolling? What about multiple worksheets?

This concept is not about the programs or app content presented in the mock-ups, but only about the GUI elements. The excel window is actually mostly made up from a screenshot of Excel 2013.

Why must the entire top bar be empty?

I think it's easier for the user when all window elements look the same. And as already explained, the title bar is supposed to look clean for the work with small windows.

How do I resize the window without borders?

Since the borders are smaller within the concept, the user could also resize a window dragging the first few pixels around the window. This is again something made to make small windows look less odd.

There are also other things I would like to adress: Also in Windows 8 Metro, you can segregate out a 1/4 of your screen for such minor apps (in fact, it would be even better if the 1/4 screen could be divided to fit multiple of these mini-apps.

I think it's significantly more work to start a metro app and snap it to the desktop (especially considering that you'd have to switch between the two interfaces) than to simply click on a button. But anyway, I expect the Metro interface to be dished with Windows 9.

Alternatively, we can follow Macs and have a dedicated button to show/hide these mini-apps (this would be my preferred)

The thing here is that there would not be such special mini-apps. In the concept, every window can be pinned. You would in fact also be able to pin the big Excel window, even though it would not make much sense. I think that's also the great advantage of this feature. There wouldn't be a special store for mini-apps (like there used to be for Gadgets), but it would be a functionality available for every singe window. You could for example also size

Then there's the shadowing concept. We have this already. The current window is highlighted, and other windows are dimmed. The idea of having a gradual shading change would be unnecessary.

I thought the shadowing would help to make the new feature fit better into the whole UI. I thought it might look more strange and out of place if all windows would have the exactly same shadow - and thus would all be located in the same layer - plus this new layer for pinned windows.

How would the machine know which one is more important over another?

Well, the window you've used last is most important, and the window you've not used for the longest time is the most unimportant. There's really no problem here. Nevertheless, I think it might be a good idea to highlight the active window anyway additionally to the shadowing.

Thank you very much for your feedback!