Color out, bland in
First, it was the Apple logo itself, going from multi-color to clearly more-elegant-but-oh-so-boring solid black. Now it seems the same is happening to OS X. While the removal of color can be seen in many apps and system widgets, to me it's most obvious--and most impacting, and not in a good way--in the Finder.
In Snow Leopard, color in the Finder's sidebar played a key role in making it easy to visually identify click targets: The Applications folder was the red and yellow A-shaped icon; my home folder was a white house, the Desktop was the purple galaxy, etc.
Now, in Mountain Lion, while the basic icon shapes are unchanged, the color is completely gone. The Applications folder, home folder, and Desktop are all now just gray blobs. And even though they have distinct shapes, it was the combination of shape and color that made each easily distinguishable. As a result, I have a much harder time finding my click target in Mountain Lion, and find myself reading the text next to the icon to make sure that I have the right one.
Where do we go from here?
As noted in the introduction, I generally like Mountain Lion a lot. The issues with the sidebar and color, however, make using it more work than it should be. At least most of the scroll bar issues can be reversed (but really, they shouldn't have to be), but adding color back isn't so simple. I have hope, though, now that Jonathan Ive is in charge of all user interface design--perhaps we'll see positive changes in 10.9 and beyond. For the sake of my overly-scrolling fingers and color-deprived eyes, I hope that's the case.