Ergos and haptics:
This thing is solid. Ergos are kinda like a slightly down-sized K-1, with some refinements. For me and my hands/ fingers, this is a good thing.
Some of the button locations have changed, which will take a little getting used to, as will the Smart Function arrangement. Big difference from the K-1, but much more customizable on the K3mkIII.
AF joystick is significantly better for my thumb than the Fuji version.
Surprisingly, unlike the K-5 I had years ago, I don't feel the urge to immediately add the battery grip. I will add one later, mainly for airshow use.
Menu system has completely changed. From what others are saying, it is like the GRIII menu. It will take me a little time to get used to it. Switching back and forth between this menu and the K-1 menu might cause me some hesitation, we'll see.
Along with the new menu, the K-3mkIII seems more customizable than the K-1. Good thing, except it does add more complexity when you are figuring things out for customization, or making changes at a later point.
Rear LCD is touchscreen. I don't use those much, but will see how this one works for me at some point.
Rear LCD is fixed. A lot of people are seriously worked up over that. Doesn't matter much to me as this is primarily an airshow/ birding camera for me. And when used for other stuff, I can make do without the flippy tilty screen.
AF System:
One of the two areas that received major attention and improvement compared to previous Pentax APS-C bodies.
Kudos to Pentax. AF on the lenses I have tried so far is a bit zippier, and less cha-cha-cha fine tuning on the screw-drive lenses.
The DA*300 almost feels like a new lens. I know the AF speed is limited by the AF motor, and the old SDM is a dog compared to the newer AF motors, but this certainly does feel a little quicker. The AF on the K-3mkIII is quicker and more decisive than the K-1. The 300 is finding focus and locking much quicker on the small birds in the tree. I'll post additional info on other lenses as I use them.
At my current health and activity levels it will take me a while to figure out all of the various AF system configurations and get the settings that work best for me and what I shoot.
OVF:
This was the other area that received attention, and it shows (ha) IMHO.
Definitely brighter, big plus.
Image is larger, inside the viewfinder. Still has a limited FoV when looking into the OVF. A problem that has spread to almost every camera. Nikon F3HP, F4, D1 and D2 series had a nice large circular viewport with little or no restrictions. Most cameras since then have a rectangular mask (I assume to block light instead of adding a decent eye piece shutter) that restricts viewing. This can be a bigger problem for some people than others.
Today, in daylight, I spent time swapping the K-1 and K-3mkIII and looking through the OVFs. Kudos to Pentax. This APS-C OVF looks as bright (to my eyes) as the K-1. And it is clear.
The aforementioned issue with limited FoV looking into the OVF. After several dozen swaps, to my eyes it looks only slightly worse than the K-1 OVF in that regard. On both cameras, if I hold the camera close to perpendicular to my head and mash the eyecup against my Neanderthal brow-ridge, I cannot see all 4 corners of the image. On both cameras, if I shift my head about 45° left and then mash the eyecup against my brow-ridge, I can see more of the image, but still cannot see all 4 corners.
Conclusion so far:
I'll need time and a lot of use to see if the OVF will be the cause of eyestrain and headaches. If it doesn't cause any issues, I think it will be a keeper due to all of the other improvements.