Looking at the posts above about safety and pick pockets, I'd like to say, let's not scare people. I have been to Barcelona (fairly recently) and I travel to European cities a lot. Sure you get pick pocketing and it is more rife that I would like, and you also have the possibility that your bag may get stolen, but let's put it into perspective, these people look for easy targets (bags left on or under chairs whilst people aren't looking, that exposed wallet poking out of your wallet, the open handbag with a mobile phone/camera showing).
I have know some people get 'stung' but most of the time they didn't even realise it - Of course this is still upsetting, but I'm trying to stop that feeling that the city isn't a safe place to walk around. For the most part it is, I personally walked pretty much from the port into the heart of the City in Barcelona and when through every sort of neighbourhood without 1 iota of a problem (in fact I felt more uneasy walking around in the dark in a UK town centre last night!).
As others have already said, just be sensible, highten your guard a little and don't invite people to make you their next victim.
Guys I recommend you keep your wallet and/or phones in your front pocket keep everything else in zippers.
Ooh and the latest craze around some tourist destinations is SLR lens theft - OFF YOUR CAMERA whilst it is round your neck - They talk to you and release the lens and off they go. Again, never any malice (as in violent behavior) from the perpetrator, just opportunity, so those of you with SLRs, maybe don't have them around your neck?
I am pleased to say I've never been stung, no pick pocket, no wallet snatch, no phone lifting, no lens snatching.
I am sure you will all be fine with just a little awareness.