Agreed with the thread title, Yosemite SUCKS. And it's not just opinion. Human vision is not set up to see all flat colors and flat objects. Humans see in 3D. The bevels of buttons and icons make them appear realistic to us, and that helps people recognize the items easily and quickly. Flat buttons with no depth create a real problem for many people to see, and it requires more dwelling on the item to visually identify it, or to identify if it is a pushable button or not.
Also the Helvetica font was NEVER really intended to be a small text font. It's x height vs width is too equal, that is the round part of e and h are the same size and it is more difficult to read the font in less than perfect or large conditions. It's not just an "opinion", it's objectively a poor choice for reading, it actually slows down recognition because it's too hard to identify characters easily.
Transparency is "cool" but the newness wears off very quickly. It doesn't really add usability to the UI/Finder.
If there was a way I could stick with Mavs UI forever, I would.