I don’t know what the new features are (reading the Apple page to “learn more” didn’t seem to reveal too much), but I upgraded my backup iMac – not my MacBook Pro yet – to Yosemite (OS X 10.10).
The appearance of Yosemite is … ugly. Everything has been flattened out and the new Finder icon, among others, look really dumb.
I think the use of slight 3D shadow effects until now have actually been very helpful in finding things. I wonder why they got rid of them. It’s just all flattened out now. I think it’s harder to focus on and quickly find icons this way.
Also they changed the colors of a bunch of standard icons: iTunes went from blue to red; FaceTime went from a grey and dark green camera icon to completely different light green box with a white square in it (?)… It seems like just change for change sake.
I wonder what’s new except for an ugly appearance…