The MISSION is the fastest changing neighborhood in SF. It is a Latin neighborhood where Spanish is the main language and most of the locals come from Mexico, Guatamala, Nicaragua, El Salvador, etc. There are 2 rival gangs that live between 17th /Cesar Chavez/Bryant/Mission. This neighborhood has become the bedroom community to the TECH industry and now a group of extremely pampered and well paid 20 something "foodies" that have descended and are slowly taking over. These people tend to not know where they are or have much respect for what was there before them. They tend to text constantly, not pay attention, drink and get mugged. It is a very vibrant neighborhood with lots of energy and stuff to do.
RULE # 1: PAY ATTENTION. BE RESPECTFUL of the locals.
RULE # 2: Hide your iPhone in public and don't walk around in skinny jeans or a mini skirt alone at 2:40am!
Wander around with friends, stick together and practice being invisible.
It's a great neighborhood in a great town. Have FUN!