To me, the greatest thing about it was that it exposed every Ribbon action with an easily discoverable and repeatable key-sequence. Just hit the Alt key once, and every top level category's hotkey showed up as a hover. Hit that key, and the commands on the ribbon (or next level dropdown) were displayed. "Paste Special", a very common action, became Alt+H+V+S. And you didn't have to do finger twisters like Ctrl+Alt+V (all held down together) to achieve it.
And again, almost EVERY command was available in this way.
Sure, you could create your own hotkeys in versions of Office before this, but the Ribbon brought almost every command within easy reach of those that hate removing their fingers from the keyboard. It wasn't quite VIM (no command mode), but it was a far sight better than anything than had existed before in Office, where most of the advanced UI had been buried away in difficult-to-automate AND navigate (either by keyboard or mouse) modal dialogs.
And by exposing most of the UI to the keyboard, it also enabled easier automation via utilities like AutoHotkey.
All-in-all, it was a power user's dream, but I definitely see how it was harder for a mouse user to develop the muscle memory on the Ribbon vs an icon bar. That, however, I believe is simply a matter of having nearly 100% of the UI be available via the Ribbon, vs. the 20% (or less) of icon bars. The always present trade-off between power and ease-of-use, at least for the mouse user.