Agreed with Bradlyhale, I have friends in a villa, even spent a NYE there (left the party early, gunshots were fired one hour later...), and indeed there are plenty of good folks. I had even talked with "pibes chorros" there who had come, uninvited, to a birthday party. They were not welcome but no way to ask them to leave. Those young guys (16/17) are so lost and dumb, that's hard to imagine. They go for "shopping" during the day, then come back late afternoon to sell cellphones and such (with no idea about the prices btw).
Had once a problem near Retiro, while coming out of the Centro Cultural Borges. I was with my -then- girlfriend, an Argentinean woman quite rich (you can guess that from the way she dresses, etc.) and skinny. Went inside a kiosco on Florida (around 8/9pmn winter time = night), she stayed at the entrance. A young guy comes nearby me to buy something. I look at my GF and there was another young guy standing 2 feet behind her (Florida was empty and it's a large street!), she had a purse at hand but didn't notice him.
Obviously the guy was waiting for his friend but was obviously interested in stealing the purse. Well, I looked at their shoes (always look at shoes to see who runs the faster) and they had running ones... I'm 40, I smoke and I had military boots on so forget it to chaserun the guy. Nevertheless I looked at him right in his eyes and looked at his friend nearby me like: if you touch her, I touch your friend... The kiosco guy realized the situation and went nervous (my GF told me since I was looking at the two guys). Nothing happened, we left and my GF told me there was a third young guy waiting a bit further. I guess they wanted to rob the kiosco but got a bit mixed up. Close call!
One thing I already knew but that those "pibes chorros" confirmed to me is that some people have "victim's faces" (cara de victima). Look around in the subte or in the bus for such "victim's face" people, they exist.
Best attitude is not to ignore those guys, not to stare at them too much, but exchanging a look is ok. If your image is "right" (like someone looking at you can be sure you're ready to have an eye chopped off if you fight), then you'll limit your risks. One thing that may favour me though is that I'm nearly 6"3 (but thin).
The French victim was Laurent Schwebel, a French geologist with a passion for photography...
http://www.grandsespaces.ch/societe-equipe.php