Well, I'd like to chip in some personal experiences. I am no way a "pro" and I'm purely a hobbyist. But street photography has always been my favourite kind of photography.
How I view street photography is that we live in such a chaotic world nowadays. In just one day for an average city dweller, on his way to work and back, he would have caught so many glimpses of so many other people's lives - all of them are interesting, but because of the information overload in modern times, we just "see and forget" these precious moments.
Street Photography, to me, is to capture these moments and emotions in pictures so we can appreciate everyone's lives in the world today.
Also, IMO, Street is the hardest kind of photography. Things happen in an instant and disappear the next moment. One must really have the instinct and attention to notice something interesting - a facial expression, a fleeting moment, an emotional event etc. These things really last for <1s. To me, it's really hard to compose, aim, focus and shoot in such a short time.
I've done landscapes and portraits, these are technically not easy, where we have to understand lighting, cloud, time of day etc to get it right. But we can take our time. Street is different - it is fast, at the moment, and most importantly, if not captured, we would probably never see the same moment ever again. That's why I think it is the most challenging.
Just my 2cents. Here's a pic
Motor-cart ride by
charpi123, on Flickr