nothing, haha. i usually don't even pay attention to a lot of the features now that they roll out a new os so frequently (i remember when they came out every few years instead so each os had bigger improvements so each one was a bigger change is what i mean). my last macbook ran on mountain lion and this one has yosemite and when i started using it, i fell in love with all of the features. i'm sure some of the stuff i love is probably not even new features, but being able to swipe on the trackpad to go back a page and stuff, basically using the trackpad as if it were a smartphone is amazing.
honestly, i really don't use a lot of features. file sharing (old as shit feature i remember using on jaguar, aka 10.2) over home network — pretty helpful since i use two laptops and also have my desktop in memphis, various trackpad gestures, i'd use airdrop if i had any other macs to share with that had the feature (my imac runs snow leopard so it's too old), and basically otherwise just browse the internet, use garageband, ms office, yeah.
metal api sounds cool, although the graphics card in my pc is apparently way better (my mac would've barely had been able to run skyrim even on its lowest settings had i run boot camp on it, for example. and i just got this last year!) so idk if i'd see much of a difference. i never notice that stuff anyway.
i like the idea for snapping two windows side by side. i know windows has had that since i think 7, not that i use the feature (i should, it's a pain switching between windows when translating).