The single most important difference between the 935 and the 920XT

Well, I don't own a 920xt and never had one but looking at it I would say looks only is enough reason already ;)

looks was the reason i upgraded. optical heart rate was the 2nd reason

I've dusted off my 910XT, 935 is down to 50 course points and won't join a course away from the start.

935 is a comfortable casual watch, might be good as an activity tracker.

I should add that my interest is function not form. It's a sports watch. I don't care how it looks. I'm trying to understand/identify what the killer feature is over the 920XT; the reason to push that purchase button.

https://www.dcrainmaker.com/product-comparison-calculator?type=watch&ids=37439%2C72457#results

That should link to a side by side comparison of the two. If not just use the tool and pick those watches. I never had the 920xt but the biggest reason i've heard people liked that watch was the square watch face (not for looks, mostly). I've only had my 935 for about a day now after returning my F5 and so far it's basically a functioning version of the fenix 5 line. Even the BT signal seems stronger. I've been able to leave my phone on the other side of the house (and through multiple walls) and not have the watch disconnect. Did a quick mile this morning (to set Vo2max) with my HRM-Tri and Stryd, not a single disconnect.

Can't think of a any crucial functionality for training that the 935 offers over the 920xt (or the 910xt) for that matter.
From your posts I know you're not interested in the following, but for others:
1. Better battery life
2. 24/7 HR monitoring which could give you a better sense of training status
3. Varia radar and varia vision support.
4. Better GUI and faster response
5. Next generation CIQ support
6. Better powermeter support
7. Bluetooth sensor support
8. Looks

I've dusted off my 910XT, 935 is down to 50 course points and won't join a course away from the start.

935 is a comfortable casual watch, might be good as an activity tracker.

Courses start fine for me away from the start (as that's how all of my courses are set up). I've used the 935 for 50+ mile rides with no issues (I didn't count the number of turns)
Casual watch? What is it that you need from the watch to make it more then a "casual watch" ? I've used the 910, 920, F3 for training and racing 3 Ironmans and numerous races in between. Each watch has gotten progressively better and the 935 has continued on that line of improvement.

From "knowing" you from many informative posts over the years, I don't think for you at least there is a compelling reason to buy a 935 if your 920 is still going strong.

I just got my out 920 to "remind" myself how it feels. Actually putting it on the first I notice on the 920 is the rather stiff and wide band which makes it feel heavier on the wrist. The 935 stock band is a bit narrower and way more flexible (largely a design to get the strap tighter for OHR) but it makes it feel a lot lighter on the wrist.

I recall you said you preferred square faces and the 935 is obviously not. That said if you overlay the 935 on the 920 you can't really see the 920 so the 935 screen is in fact similar size and I find the screen somewhat more sharper although never found the 920 hard to read.

Turn the question round and what I do prefer.

Strava segments - use these a lot (perhaps too much sometimes!)
Has Golf app so all my main sports Running, Cycling, Golf are covered by one watch for first time for me in a Garmin have owned
Feels lighter and sleeker - hardly notice have had it on and I usually wear it to bed.
Training Load & Status I find quite interesting and motivational - e.g. can I keep being "Productive" for a few days etc etc
Accessibility of all watch features even with the timer going (e.g. accessing widgets mid run)
Best navigation have had in a Forerunner including pan & zoom of the map and ability to upload raw GPX files and convert to a course automatically
Almost all the features have ever used in a Forerunner plus some probably have not found yet!
As many data pages as you can handle
Connect IQ future proofing
Does the basics fine - e.g. GPS accuracy is decent, current pace from GPS not too bad, distance from GPS not too bad. 920 was the same though. Only sensor have problems with is my Rotor PM but the 920 was probably worse and it works fine from my top tube bag if not my wrist when on aero bars.
Keeping up with the Joneses!

Best navigation have had in a Forerunner including pan & zoom of the map and ability to upload raw GPX files and convert to a course automatically

That's a good one. I use courses a lot over here (I'm in Aus researching for a couple of years) for getting to places off road. Garmin Connect works well enough for me but I'd like to be able to use raw GPX files, especially if I can get them from someone's Strava or GC track.

Keeping up with the Joneses!

That's been my usual rationale but the 920XT is such a good watch that I really need another excuse to move this time.

The screen is definitely sharper. I've gotten to the point where I actually prefer the round watch face. Fields like time work great on the top/bottom where you have more width, HR and distance work just fine on the middle fields. CIQ custom full screen fields taken even more advantage of the screen real-estate.

Also played with the navigation for the first time today and was impressed with how easy it actually is to zoom in/out and pan without a touch screen. Combined with the higher res screen makes that map page that much clearer.

Size/weight/comfort is another huge one for me. Sort of falls under "form". I too could care less about what it looks like. Although I do think it's the best looking Garmin out there.

Quick release combined with quick fit bands. Makes it super easy to mount on the bike (I've started using it instead of my Edge 810) but also lets you easily re-attach a regular quick fit band in seconds. Always hated the extra thickness the quick release bands have added in the past.

Easily the best Garmin I've owned. By far the most stable Garmin I've used out of the box.

I should add that my interest is function not form. It's a sports watch. I don't care how it looks. I'm trying to understand/identify what the killer feature is over the 920XT; the reason to push that purchase button.

Fair enough of course ;) But it's kinda hard to name a killer feature, especially for someone else...

But for me one of the biggest reasons would be:

1. wrist OHR: I just hate the chest-straps. Then again this really depends on how precise it needs to be for you and what your main sport is (ie for running, which I just about solely do, it's good enough)
2. For me honestly, looks... I wear it as a daily watch. So after owning the tomtom I went for garmin because I could actually see myself wearing that 24/7
3. up to date connect iq support, depending on how much you use it of course.

But all in all it's just the entire package, everything has been updated and upgraded... It's not just one single feature.