Look of lens. I tried for years to reproduce the look / tones of photos in the paper sample books in all the camera stores. No luck. I had 12 Pentax lenses all new and current.
Then I was loaned a Leica . Same film batch, same developer, same paper and there it was ! WOW.
Then I found a 125 mm Hector and cobbled up a mount for the Pentax.
I did a roll of SLIDES with flash at a party with photos intermixed Takumar and Leica.
Sold all the Pentax and bought Leica.
Even today the Leica lenses look better on my D800 than Nikons current glass.
When I shoot a color checker, look at the reds. Leica reds look like apples, Nikon AiS and Ai and current all make reds look orange.
Dond`t take my word for it. Go to Imaging resource Comparometer and compare the M9 with any Nikon. Use the image the studio set up containing wine bottles and crayons and yarn samples. Look at the red yarn and the color checker. Red is third from left above the greys.
When I do an outside shoot, Nikons greens are are over saturated and photo is lowish contrast. We are talking SAME camera here, just changing lenses.
If you want to really be shocked, use 100 2.8 APO on the Nikon.
Conclusion is Leica lenses are obviously superior, not in sharpness totally but the look.
Zeiss lenses from `80 s were also nice. Not as nice as Leica, but nice. They were not as well made either. CV makes the new ones. Don`t trust CV for various reasons.