If Sony has truly succeeded in cramming the power of a pro action dSLR into the body roughly the size of its A7 series of mirrorless cameras, that's a big deal. Smaller cameras mean smaller lenses, which matters when you're using a huge telephoto lens. Sony also brings in-body image stabilization, which is a lot more practical than the optical stabilization in the top dSLRs.
One of the big problems with shooting action through an EVF is that you're always seeing what just happened rather than being able to see in real time the way you can with an optical viewfinder. The A9's refreshes at 60fps -- it can go as high as 120fps -- at high shutter speeds, which the company claims is sufficiently fast to eliminate that weakness. Resolution is 3.7 million dots.
Frankly, all the A series cameras should have dual slots. Only one of the slots supports UHS-II, though, which may be a drawback. It's ironic that Sony, which was the driving force behind the XQD standard, still doesn't use it. (It still supports Memory Stick, though!)
Photographers used to the electronic implementation for exposure compensation may not really want an analog dial. More important, not having a top status display might be a sticking point for some, though you can see the same settings in the viewfinder.
Like the other cameras that used stacked CMOS sensors, the A9 has built-in slow motion (the S&Q on the mode dial). It doesn't reach the frame rates that the smaller-sensor models do, but it can do from 1 to 120fps in HD resolution for up to 60x faster or 5x slower. In PAL, it's 100fps at 50x faster or 4x slower.