What I think Keith has noted is that the higher resolution images actually have more detail, clearly observable with a loupe or using a macro lens. On the other hand it seems that the extra detail doesn't matter to viewers. On the other hand viewers can pick up thinks like differences in global contrast and small differences in composition.
This is pretty much what I have seen comparing prints in A2 size from my 24 MP and 39 MP cameras. More detail in print but little perceived difference.
My guess is that we see is that the human vision is most sensitive to medium frequency detail. The eye can resolve fine detail but medium frequencies will dominate perception, see linked figure from Wikipedia. So if we discuss detail visible at 360PPI at 25 cm, the contrast sensivity of vision is several magnitudes below maximum. To me that indicates that we should focus our sharpening efforts more on medium frequencies than actual pixel detail.
What I have observed that 12 MP was pretty good enough for A2 size prints. At my recent exhibition I had a mix of 12 MP and 24 MP images, reflecting the state of shooting irons in my possession at that time. On most images 12 or 24 MP mattered little. Could be that 24 MP or higher would make a noticable difference.
But, on one images there were a lot "dust specs". I realised that those were birds and not dust, but 12 MP was not enough to resolve them. Now, 24 MP may not have been able to resolve them either, that is just a 41% increase in resolution.
The impression I got was that the visitors placed that image #2 of all pictures at the exhibition.
This was #1:
Do note that the prints IIRC were made at 300 PPI, and maybe output sharpening was (therefore) not as good as is possible when one does have that extra, 4x as much, detail at 600 PPI to sharpen with. Of course relatively casual viewers will look more at the image itself than the technical quality of it. They will subconsciously 'experience' a difference but will not be able to put that into words.
It's a burden I happily carry with me all my life, but technical image quality is important to me because, besides showing attention for detail in preparing the image (and thus taking the viewer serious by not holding back), it does work on the subconscious observation. Material texture adds 'life' to the subjects we image, and if done well, it's not readily noticeable but it's present, as if subjects become more tactile, but not harsh.
I remember when I did my exams for my Photographers license (was mandatory for settling as a professional photographer many moons ago), and one of the examiners who had to judge my assignments looked at my assignment 'glass'. I had shot many different subjects with glass as the main motif, but finally settled for one beautiful stained glass window. The examiner looked at the large print that I had submitted and asked me with which view camera I had taken the image. He then turned over the printed image with my negative attached to the back in a sleeve, looked at the tiny 35mm negative, and said: "Okay, next assignment ...".