How is that any different from the situation on OS X Mavericks and previous versions? Spotlight has always been blue, same goes for Wi-Fi. Parental Controls has always been yellow. All new icons share the same new matte cartoonish look that's what binds the FaceTime one to Calendar. The System Preferences icons in DP7 have been given an overhaul to be more consistent compared to DP6 / PB 2.
I think what I am trying to say here is that Mavericks did not have the ambition to be minimalistic. The icons were certainly not consistent, but it did not really matter, because they did fit the big picture. Icons in Mavericks are mostly images (in semiotic terms), that is, they visually represent the object tor action they point to.
In Yosemite, some of the icons have been reworked in a minimalist style, while others have not. The problem here is once you start going minimalist, you'd better see it all the way though. And being consistently minimalist is much more difficult than many or our friends, the 'flat look critics' seem to comprehend. Again, when we talk about semiotics, icons in a minimalist style have more common with diagrams and metaphors than just images.
Basically, once the icons have been redesigned, one needs to ask the question as to the purpose of the redesign (they could have left the old icons and it would still look quite ok with Yosemite, wouldn't it?) A minimalist redesign usually involves some kind of red thread, a core design principle — say the grid system so widely praised by Apple. So far, some of the Yosemite's icons appear to follow the grid system, and some not, some use the iOS design and some not, some seem to be color coded and some not. One of the core ideas of minimal design is to establish visual invariants (elements) that help the user to learn the purpose of the interface items. Yosemite performs quite admirable on this front, IMO, but it fails short if its icons design is considered. I'd prefer either a more heterogeneous design or a more consistent one.
