There are so many things about the a9 that Sony improved over previous cameras...better viewfinder, AF selector joystick, better ergonomics, bigger battery, dual card slots, touch screen, improved tactile feel of dials and switches, better menu system, and on and on. These are all important things that Sony was playing catch up on. But I'm going to focus on what I think makes this a revolutionary camera instead of just an evolutionary one.
For a sports photographer, no viewfinder blackout is a paradigm shift. And I didn't realize how much of a shift until I went back to use my a6300 and a7rii. At first, not having viewfinder blackout seems kind of cool but not that big of a deal. But after using it for a while, it becomes seamless, and a huge advantage for following erratically moving fast subjects like skiers or soccer players. That moment of viewfinder blackout that I grew accustom to was often just long enough and would occur at the wrong time for when a subject changes speed or direction. As a result, there were often shots where the subject was partially out of frame or out of focus as tracking them during the blackout was a process of expecting them to continue on the path or speed that they were before the blackout. With no blackout, tracking them is much easier. So it was subtle at first shooting on the a9 but before I knew it, I was really using the no blackout viewfinder in ways I wasn't aware of. Picking up a traditional mechanical shutter camera felt like picking up an antique. Suddenly, everything else felt like old technology.
There are two additional features of this camera that make it groundbreaking: autofocus and image quality/dynamic range. Mirrorless autofocus has been getting very good, but hadn't managed to be as good as dSLRs for focus tracking a moving subject, until now. That's completely changed with the a9. The AF tracking on the a9 is as good as anything I've used, with the previous bar being set (in my experience) by the 5D Mark III. The a9 is as good or better than the 5DIII. It's hit rate is incredible and the camera acquires focus seemingly instantaneously and effortlessly.