Yosemite leads to serious framerate drops when connecting a rMBP to an external display like for example the Thunderbolt Display.
Here is a video that is a good demonstration of that problem:
https://www.youtube.com/watch?v=tktaJuD9JK4
Jonny Ive wanted to get rid of skeuomorphism and now introduces ui laggyness caused by translucent material (which stands against the purity paradigm IMHO).