Extraordinary lenses (especially the new 1.4 versions),unbelievable bokeh, small, quiet (M7 makes only very quiet "tsss" while shooting), absence of mirror-with steady hand I can use 1/8 and 1/4s with wideangles (with practice I use to shoot even 1/2s hand held) - this is no chance with mirror cameras which make noise and shake like Russian Katyusha.
For large percentage of my shots I use "blind shooting" with camera hanging on my neck or even hidden in the pocket. I took all my shots from inside mosques this way and lot of Cuban shots as well. I tried to shoot a fire of petrol or diesel storage tank in Cuba and I made couple of good shots "from the pocket" while arrested. When Cuban police confiscated the film from me I was able to change it and gave them empty film.. .
Leica M is without doubts absolutely the best camera for street photography ever. It's not a hype, it's holy truth.
P.S. I'm not paid by Leica. I had to buy all my gear and had to pay a hell of a lot of money for that..