How big is it?
What is he thinking?
Where is she going?
What has the dog got?
Ambiguity is an important element of "street", it's pretty much all there is in "abstract", and it appears here and there in all the arts. Used with care, it is a potent tool.
Viewers, to varying degrees, enjoy a puzzle, a mystery. We enjoy being allowed to fill in pieces of an image from our own experience and imagination. Given something unknown, we write our own narratives and develop our own ideas about an image. To a degree, we enjoy things simply left unknown and unknowable, but hinted at. We may not be able always to hang a story on those elements, but we like them.
The risk the artist runs in using ambiguity is in losing the viewer with an excess of mystery. A viewer might understandably resent being required to work too hard to make sense of an image, and thus move on too quickly. A viewer might stick with an image, but simply find it baffling. While bafflement is certainly a reaction to an image, it is rarely what the artist had in mind. While every viewer will have a different threshold of "too much" mystery, surely the artist wants to calibrate the mystery to a good level for a lot of viewers.
One approach to managing the degree of mystery successfully is to wrap the ambiguous up in a generous serving of the familiar, the easily grasped. It is this, I think, which makes a particularly successful "street" photograph. We understand much of the scene, we recognize buildings, people, the dog on the leash. It is only a small subset of the image that is ambiguous, ".. but what is SHE doing?" A mystery or an incomplete story given to us as a reward for looking a little deeper. It is this wrapping of the mystery in the known that makes a great portrait, as well. We recognize that the image is a portrait, we recognize many of the elements. Somewhere in there, often in the subject's expression or body language, we see elements that are unclear, ambiguous. On this ambiguity, we write our own interpretation.
See also: the Mona Lisa.
Poorly balancing the mysterious with the familiar can lead to images which are either banal, or too difficult to understand. The viewer moves on, the message is lost.
When we abandon the familiar, we enter the realm of the purely abstract, for all practical purposes. This is a tough world in which to create an effective image.