I continue to hate the Ribbon UI. The biggest problem is that it is nearly impossible for the average user (even advanced, really) to customize for their own preferences. I'm pretty sure that in the first version of it (2007?), it was completely impossible without being a developer.

And while the old UI would change things around a little as context changed, the extreme way that the Ribbon does it is really irritating. Things appear and disappear even if you just resize the window.

It also seems that, if they haven't been reduced altogether, it's very difficult to find keyboard shortcuts.

Also, not being able to see your document while you have the File tab open is awkward.