The one thing I'm not a fan of is the horrible, and absolutely terrible animation to take windows fullscreen. It takes a proper 2 seconds for a window to go full screen. I just can't see the reason behind this. At least provide an option to disable this animation or provide a terminal command to speed it up like the mission control animations.
Handoff is one feature I use more than I thought I would. It still feels like a novelty but I'm glad it's there. I use it most to continue my emails on my Mac from the iPhone.
Airdrop on both iOS and the Mac is simply amazing. Now I don't have to wait for Photostream to sync between the two devices.
And the little things like Javascript for automation, built in actions for batch renaming and encoding media files inside the right click contextual menu, the dark mode all add up.
I'm also looking forward to Siracusa's multi-page review about Yosemite. He hasn't been a fan of transparency, so it will be interesting to hear his thoughts on it. I'm not a fan of it either but I like the transparency used in Yosemite. It's not as distracting and the dark theme really helps with that.