I find it interesting that so many people comment on the unique Noct bokeh, when at least to me it looks very much like what you get from a Sonnar.
But what is unique to the Noct is the nearly total lack of flare. There simply is none and as Erwin points out in his review, there is so little scatter as the photons travel through the elements, that pretty much only the light contained in the scene reaches the film. To me that is what gives Noctilux images their unique look. Unfortunately this was one of the main reasons why I sold my Noct. I felt that the images were flat and I prefer a little glow and blooming. The images from the Noct are so perfect, that they contain none of these traits.
Just to give an idea of how flare proof this lens is, here's an example. One night I was on a night shoot for a movie. To entertain myself between takes I shot directly in to a 20,000 watt movie light, that was suspended about 20 ft off the sidewalk, The Noct didn't even bat an eye. When I examined the frame I saw geometry (Octagons etc) from a lens flare, but no actual flare. Pretty amazing.