The Dock absolutely lost usability; all the icons look the same now. I practically stopped using the dock because I never find the apps I'm looking for quickly; I've switched to using Spotlight for opening apps instead.

The old "Pages" icon was instantly recognisable. The new one looks, at first glance, exactly the same as text edit and notes.

I don't think that the UI actually needs to get out of the way. We humans are perfectly able to ignore even the most obnoxiously designed mess (cf. banner blindness). But we are not very good at picking from many similar-looking things. Replacing the colorful sidebar icons with simpler monochrome versions now requires us to actively look for the icon you need, instead of just picking it intuitively.