While the Ribbon UI was intended to simplify complex UI, these screenshots suggest it may not. Take for example the "Explorer in Windows 8" image. The visual hierarchy is very confusing. The ribbon appears to have 5 pages - but File is bold and special for some reason (it invokes a huge popup menu?). But the focus of the entire app is on Files - so what conceptually would not go under File, except for Folder operations? Furthermore, above this special File menu there are three super special (and somewhat inscrutable) icons. But aren't these just shortcuts for File operations? Why don't they just live under File menu? For that matter, why have toolbar buttons been hoisted into the title bar itself?
The current tab appears to be the Home tab, based on the highlight. But this has nothing to do with your "Home" directory - it is merely the "default" page for the most common operations. These operations are used to manage your files - the names of two other tabs in this hierarchy. What would the user expect to find there?
The layout of the Home tab is visually confusing. There is no grid or use of whitespace to guide the eye, and it presents a non-uniform 2-dimensional array of controls of different sizes. At least the old toolbars being 1-dimensional meant the user could easily remember approximately how far along the toolbar to look for the icon - now the user must scan a far more complex terrain to find what they're looking for.
There are two different sizes and layouts for action icons. So Copy and Paste are large icons with text below, while Cut is a small icon with text to the right. Why? Presumably it is less frequently used, but the inconsistency is visually jarring and non-obvious to the user.
While some buttons cause immediate actions, others have a small down-arrow next to them, suggesting they will probably pop up a menu instead. This introduces another variation to the two button types, giving a third type. But some have a default action when clicking on the button itself (a fourth type), and only reveal a menu when clicking on the arrow - yet there is no visual indication of which behaviour a button has. (This is apparent in IE and Office also.)
A user may wonder what is the difference between clicking on the dark blue File toolbar tab, versus clicking on the menu button in the very top of the titlebar, versus pressing Alt to reveal the hitherto hidden menubar?
While the Manage ribbon tab appears at the same hierarchy level as Home, Share and View, it has a brightly coloured tab _above_ it labelled "Library Tools". Vertical lines suggest it is superior to the Manage item; yet presumably it is also higher than Home, given its placement and urgent red/orange background? What is the difference between pressing Manage and selecting Library Tools?
A user not trained in the intricacies of the Office Ribbon UI will IMHO be utterly baffled by the complexity exposed in this UI and the inconsistencies therein.
While it is laudable that Microsoft is attempting to refresh the design of this venerable component of Windows, this work seems to be little more than a reshuffling of features and shortcuts, rather than a rethinking of file management.