I've been to Barcelona twice. A beautiful city that I really enjoyed. I never felt threatened in any way (but see next paragraph). I went out at night to local clubs and didn't feel that there was any more risk than going out in a large US city. Take the same kind of precautions, ask people before venturing into areas that you are unfamiliar with at night.

Las Ramblas is a large interesting boulevard through the middle of town and is a lot of fun to see. However, in any location where there are tourists, there appear to be very talented pickpockets targeting them; be careful.

I'm an amateur (very amateur) magician so I was fascinated to see the street scams based on the classic shell game being performed openly on Las Ramblas, complete with confederates winning money etc.