You learn a lot about people and crowd dynamics. Donations come in clusters; if a person sees someone else donate, he/she is more likely to. In an attempt to teach empathy, mothers often give cash to their three-year old kid to put in the violin case. The few who drop a $10 or $20 are often over 40 years old.
Twice I've had money stolen, both by dudes in their late teens or early twenties.
Sound definitely matters; you can't just "look good". Busking at the Ferry Building was a bad idea; no one can hear you. I'll never do that again.
High-density foot traffic is not always a good thing. In the evening rush hour, there are so many people obstructing view and making noise that I imagine it's difficult to appreciate the music by the time you've reached the fare gates. I believe that a medium traffic density works best where the sound can travel and there is always a direct line of sight between pedestrian and performer.
Sometimes the "great works" make poor busking choices. The Bach Chaconne is a great example.
Unsurprisingly, an ensemble with more performers earns more than a soloist. On August 20, 2014, we played the Mendelssohn Octet in Powell Station and collectively made $461.64 over 90 minutes, or $307.76 per hour. However, that's $38.47 per hour per person. I make $15-50 per hour solo. So I wonder where the diminishing returns begin, i.e. what ensemble size maximizes revenue per hour per person.
Is it scary? The scariest part was leaving the apartment that first day. Exiting the apartment door was the point of no return. "Okay, I have my jacket and shoes on and my violin, I might as well go on with it." That moment when, for the first time, alone, I plopped my music stand in the middle of Embarcadero Station was one of the most uncomfortable moments of my life. However, once you start playing, that's easy. The music is the easy part.
If you want to busk but are scared, I recommend going at a less busy time, e.g. Saturday morning at 10 am. Get used to the surroundings; embrace your anxiety. Once you're comfortable, try again next week at 2 pm.