You may find it hard to believe Christoph but it doesn't make it any less true!
I no longer bother with Nikon stuff but I have years of files from the D800 and E, not the D810 and I can spot an S file easily, there's not one individual thing, it's a combination of many things that give the S simply better files for the work that I do. I doubt it's universal, we don't all see or value things in the same way. It's not about convincing someone that they are right or wrong, proving anything, it's about finding what works for you regardless of what that equipment is. I don't think it's important that anyone else sees what I see in my files, why would it?! I don't personally have any wish to persuade people that the choices I make are the only right choices and I certainly don't need to prove it unless you are a client of course!
I have said before about the S compared with Nikons, it's like taking a D810 adding a huge viewfinder so that you could accurately focus Otus lenses wide open with little problem. then getting Zeiss to add af to the otus, expanding the range to add the same quality to all lenses and then add a sensor with larger pixels, smoother transitions, and an incredible 3d feel to the files. That's how I see it, it makes the S worth every penny and more! I have earned more money in the last 2 years from the Leica purely because of how it allows me to work and the files I provide to my clients.
As long as you enjoy the kit you have then that's great, more power to you!