I'm amazed how a dinosaur like question can evolve into something good. Everyone here as added a great opinion. Two years ago If I had read this question the obvious thought would be a lens that can operate a full frame camera. A weaker support system of fellows would agree simple behind the fact that they can afford that type of camera system. There are other factors which support a bigger camera with a 50mm or 35mm unit attached. As I've learned its a question of sociability status, or some form of Impressionistic deception to your ability to nail the true intent of a shot on to your camera sensor. Wither you are in stealth mode, or just sticking out like a sore thumb, the moment you press the shutter button something happens between you and the subject with your camera and lens of choice.
We tend to forget how comfortable we are at home in our city, state or country when thinking about street. You can relax , pick up calls on your iPhone, listen to music and text as you go. We can be a connected breed of shooters. To supplement this thought, one has only need to check into a Starbucks, or restaurant for some social chow. Time passes and you are ready to go it the road shooting or call it a day behind light shooting. As to appose traveling to a different country you find yourself on your feet for the entire day with uncertainties to find a good restaurant you can trust or a Starbuck like coffee place. A big camera and a heavy lens will turn your street shooting into a hell week. Once fatigue sets in to your limbs You can really begin to feel the muscle burn while lifting a big camera.
becomes slow and you find yourself late on the shot. If it gets real bad you just say "that's the one that got away".
I believe there are only two good cameras to shoot with that are small. The Nikon 1 V1 has been a camera of choice for he past two years.. It fits in the breast pocket of my shirt if I use the 10mm f2.8. Lens. Another small camera and lenses I work with is the Om EM5 and E-PL-5 from olympus. The low ability to expose and focus is unreal and exact should you use the touch screen. It's a no fear camera. However you need the 12mm f2 or the 17mm f1.8 attached. The glass is equal to the Nikon Nano glass found in their 24mm f 1.4 and 35mm f 1.4 lens at a fraction of the cost.
The Nikon 1 V1 ( N1 V1) has a different feel with its 10mm lens that is equivalent to a 28mm for a full frame camera. In normal light the N1 V1 has no weakness. In low light You really have to be patient and hit the moment before the subject allows his body to be one with his, or her moment into the law of kinetic energy. I only submitted one image of a girl lifting a bucket of water in india. It's a good example for twilight shooting or low light like conditions.
If any one asks why not the nikon 1 v2. The answer is simple. Nikon just missed the boat by building a bigger camera that requires you to work with the side grip on the camera which causes it to loose it's stealth factor for camera handling.
Robert mentions the 60 frames a second. Its a cool concept. the Ni V1 also offers 30 frames and 10 frames. The painful thing is that the camera locks into an auto mode which cranks your iso to 3200. Sorry Nikon can't punch it's way out of paper bag made in China with on glue on the seams at that high iso. Beside 1x is not a big fame on camera noise from an extreme high iso level. It's best to work the camera in manual at a lower iso and pray you nail the shot. However if you rely on the remark ability of the camera's electronic shooting mechanics you could be looking at a few hundred images of the same subject with very minute differences out side of the fact the image is out of focus. With that said we can all imagine going cross eyed looking for the one image to submit jam-packed with noise.