The biggest problem I have with the newer versions of Windows 8, Office 2013, and even Windows 7, is the lack of Visual Distance between elements, making it hard to distinguish between items. The Windows XP Theme did not have this problem.
Example 1 - Spot which task is active
Here's one example from Windows 7 task bar: which application is active? Can you spot it? No? Not easily? That's because the colour difference between the active task and the inactive tasks is very small.
The active task was actually Google Chrome, as shown in the red box (you may have to scroll right to see it).
Example 2 - Spot the top-most window
Can you spot the window that is top-most in this screen shot from Visual Studio 2013 on running in Windows 7?
Can't spot it? It's here, inside the red box:
Example 3 - What pane do these tabs or sliders belong to?
In a complex multi-window tool such as Visual Studio 2013, it's hard to see where one pane starts and ends, and so it's hard to tell what sliders belong to what pane. You have to remember that sliders are always on the bottom or right; the interface no longer tells you this visually.
In the example below, there is no indication whether the tab elements or the sliders belong to the pane below them or above them.
With Windows Aero, and see-through windows turned on, it was impossible to tell which window had the focus because the window bar of each window changed colour depending on what was behind it at the time. Thankfully Microsoft made it possible to turn that feature off.