The heft, the balance, the location of the controls, the smoothness of operation. The appeal a Leica M has is not much different than that of a firearm.
I remember the first time my grandfather let me fire a handgun. He was a former cavalryman from the days when the cavalry still rode horses and carried sabres, his title being "stable sergeant" His Colt Army revolver was quite old even when he acquired it, and I had always lusted after it when I was young.
When I was allowed to handle it, I remembered how heavy it was, the way the grip felt in my hand, the natural balance it possessed, and how wonderfully smooth the cylinder turned upon it's spindle. It was a wonderfully designed and made device, supremely effective in operation and use. Much like a Leica M.
In my grandfather's day, the old single-action Colt was already obsolete, being replaced by semiautomatics. But many troopers, like my granfather, much preferred it.