I've beenvery eager to find out what has happened to the user interface of the E-M10 Mark III, so I have sacrificed some of my money and sanity (what little there is!) on one to investigate more thoroughly. Please bear in mind that this is an initial look, so probably nowhere near complete. I've skipped video because I am so terrible at it that I don't generally use it on any camera, but of course I could try to answer (easy) questions about it. I will mainly compare the Mark III with the E-M10 Mark II as I go along. I think it might get a bit long-winded, so sorry in advance!
Physical aspects:
Mine is silver; it looks very nice (like an E-M10 II crossed with an E-M5 II). It's painted silver (shades of the E-M5 II, but less blue/grey) but the top and base covers are polycarbonate, not metal, for the first time in an OM-D. It's hard to tell actually (feels fine, same weight as the E-M10 II) and will be even less obvious on the black version, which is speckled like the black E-M5 II.
The mode dial is slightly looser detented than on my E-M10 II, whereas the control dials are a little firmer than the E-M10 II's. The power switch is larger and a bit lighter in action.
Same shutter as the E-M10 II, from what I can hear.
It has the extra contact in the hot shoe for the FL-LM3 tilt/swivel flash, if you have one (the E-M10 Mark II also has that).
The enlarged grip feels nice; the rear thumb grip is a bit better for me than the Mark II's. As noted by others, there isn't an ECG accessory grip for the Mark III.
First time for an E-M10 model: it has the tougher, smoother-action lens mount usually found on the more expensive models (looks duller, warmer in tone). This should avoid the stiff action of the previous E-M10 models' lens mount with some metal mount lenses.
Onto the user interface. I'm afraid a lot of this covers missing facilities. This first part refers to the menus and facilities in P, A, S, M modes. The style of the menu system is now like the E-M1 Mark II (it's hard to describe the difference but it's less colourful and slightly more linear). The menu pages are a LOT shorter and that's because a lot of configuration options no longer exist (to those who wanted simplified menus, I could say be careful what you wish for!).
If you happen to like the "non-live" version of the Super Control Panel (where the SCP is permanently displayed on the rear LCD with a dark grey background, and you primarily use the EVF for shooting, somewhat DSLR-fashion), I'm afraid it's gone.
As well-publicised, there is no longer a flash RC mode and interface.
There are NO MySets, and NO ability to save any multiple preset customised modes to anywhere, including the mode dial.
You cannot turn off the Mode Guide that flashes up a superimposed description of the mode you just selected on the mode dial (though it does clear if you half-press the shutter release, and doesn't hang around for too long).
There are no longer any super-small square AF points (with the little S in the selection screen) - this is a change shared with the E-M1 Mark II, along with the 121-point (versus 81-point) AF array. The E-M10 III doesn't have that camera's 5-point cluster option (just single, 9-point and all points), and it doesn't have a Home Position (HP) setting either. When using all points, the E-M10 III shows all the points that have locked onto a particular distance, and keeps them lit. It only does this in S-AF mode, though - so no "dancing squares", even though it looks like a static version of that mode on the E-M1 II. You can still use "zoom AF" (via magnification button, either actually zooming the image or not) to make a smaller single AF frame.
Auto ISO setup is the same as on the E-M10 Mark II - no additional tweaks as on the more recent, more expensive models.
Surprisingly, there are no optional EVF styles, just the one that matches the LCD (which was Style 3 on earlier cameras). Consequently you cannot have that option that pops up the horizontal level gauge on half-button press in the smaller, DSLR-style EVF display options (Styles 1 and 2) since they don't exist any more.
There are only 2 customisable (Fn) buttons - the one at the side of the shutter release and the one atop the rear thumb grip. The list of functions assignable is considerably shorter than on previous models (ask if you want me to list them). Electronic zoom of lenses cannot be assigned to control buttons, for example. The 4 multicontroller buttons are permanently assigned to their marked functions (they cannot be used for directly moving the AF point) and the red movie button is locked to its function. The button adjacent to the power switch (Fn3 on the E-M10 Mark II) is now for bringing up context-sensitive main menus (in P, S, A, M it brings up the Live SCP; by default, the OK button brings up the Live Control menu in these modes, but it can be altered to bring up the Live SCP as well).
You cannot disable the half-press activation of the IBIS system, unlike all previous models.
You cannot choose between 1/2 or 1/3 EV steps for the exposure and ISO systems - they are permanently set to 1/3 EV.
There's no longer an option to reverse the focus ring direction.
There is no "hold" option for depth of field preview - if a button is assigned this function, it must be held in for use.
There's no S-OVF option, and no Multi-Function setting for an Fn button.
Dial rotation cannot be reversed, and dial options are considerably pared down - just Ps and exposure compensation swapping in P mode, aperture and exp comp swap in A mode, shutter speed and exp comp swapping in S mode and aperture and shutter speed swap in M. No swapping at all in playback.
There is no longer any option to select release priority - the camera will not fire in AF if focus has not been achieved (I think they've been paying too much attention to how Canon mirrorless cameras do things! ).
No full-time AF option (i.e. battery-wasting pre-AF - did anyone use that?) or normal/short shutter release lag option.
Some settings that don't appear in the menu system or the default Live Control interface (which is the Canon-style menu bars that appear across the base of the view and up the right hand side) ARE available in the Live SCP (aspect ratio and Highlight/Shadow curve adjust, for example). And of course the Live SCP has some touch functionality, which the Live Control interface doesn't.
It is completely impossible to access any form of bracketing in PASM modes. Weird decision.
It is completely impossible to access silent shutter settings in PASM modes. Another weird decision. Both of these are only accessible in the new AP position (Advanced Photo) on the mode dial.
I'll try to cover the other operational modes in another post...
