I don't think I have ever had a digital Leica in my hand, so my capacity of judgement is limited. Just a few points:
1) The Leica M is, as far as I can see, a compact viewfinder camera, therefore not quite the same beast as the d800 which compares more to the Leica S, also regarding the pixel count.
2) What is the dynamic range of the Leica? Is it anywhere near that of the d800?
3) As stated, I lack the first-hand comparison, but I never felt the d800 to be a heavy camera, and 230g does really not seem much to talk about. I do not doubt that the Leica is very handy and probably somewhat more compact, though - after all it doesn't have a mirror.
4) The Leica prices are so high that the camera has to be of first-rate quality - you can fool people only so much. On the other hand, there are lots of people who buy Gucci suitcases and I-don't-know-what shirts just because they want to buy the name.