While many people find the inclusion of the Ribbon in Windows Explorer debatable, I don't think the Ribbon is a failed concept. It's excellent for its purpose, and that is to provide a) an accessible user experience for new users b) versatility for experienced users and c) swiftness for really experienced users.
Point a), given the intuitive interface, is more or less a given. Point b) is the most common source of disagreement among users, others say it hinders their ability to work and the others say it makes it easier, because they find features they have never seen before. The latter makes sense, as that was one of Ribbon's purposes. The former is a matter of getting used to, and in fact, I will elaborate on point c) in this regard. Point c) is about key bindings. Yes, key bindings.
I mostly do text editing and programming with vim. So I live and breathe keybindings. The Ribbon UI is designed to provide dynamic keybindings for everything. You simply press Alt and the keybindings will highlight above the buttons and tabs, highlighting subgroups dynamically as you go, sequencing tab groups. For example, in a hypothetical Ribbon program, if the 'Insert' (I) tab group had the subgroup 'Image' (M) and 'From File' (F), one would press Alt+I+M+F to access this option. This is extended to every control in the application, and it allows everything to be keybound, requiring no mouse input, which I find slows me down. So if anything, this will make using Windows Explorer faster for the experienced user, provided he is willing to learn keybindings (or just watch the labels).
Another strong point about the Ribbon is that it can be hidden. Towed away, able to be called back with a keybinding. Thus if one finds the Ribbon obtrusive in anything, one can effectively minimize it -- making any Ribbon UI more minimalist than its previous non-Ribbon incarnation!
So speaking as a "power user" of applications I, for one, find the addition of the Ribbon to Windows Explorer a pleasant surprise. While I do not feel its inclusion to be completely warranted--what does one need from a simple file manager anyway--it will make using the program a lot faster for someone used to having keybindings for everything. I'm sure most of you can relate to this sentiment.
"Today's robots are very primitive, capable of understanding only a few simple instructions such as 'go left', 'go right', and 'build car'." --John Sladek