Below are my four initial impressions of Apple's latest desktop OS, displayed as bullet points for your convenience. I even threw in an extra one (like the black keys) for free.
My slightly-OCD side appreciates the tightning up of the system, rather than emphasis on new features. I wish they did this every time, ala Intel's tick-tock.
San Francisco as the system font is... regrettable. I'm not a typography professional, but I like to dabble. It looks childish and unrefined, and is what bugs me about the Apple Watch. At system UI font sizes, Lucida Grande was still the most legible.
DiscoveryD appears to be gone for good. Introduced with Yosemite, it's been responsible for the flakiest networking and sharing performance of any OS X release I've ever used since the 10.0 betas. I've got so many duplicate (3) printers it's ridiculous.
I no longer use nano, but it's my test to see how often they update their UNIX userland. Yosemite still shipped with 2.0.9, released in 2009. Let's see if El Capitan comes with 2.4.1.
Apple continues to troll pedants with their names. 10.9 Maverics got "awkward pluralisation", 10.10 Yosemite was mispronounced by everyone who'd never grown up watching Warner Brothers cartoons (really?). I'm already looking forward to seeing what they name their next one ;)