A couple things to look at:
-does the hostel have a private room or are there only public ones?
-is a private bathroom important to you?
-the bigger the room if it's public the more weird crap that will go on at night
-is there a curfew/time that they lock the door
I've stayed at several hostels abroad which have all been pretty nice and for the most part if you don't want to talk to other people they will leave you alone. A private room is worth the extra few bucks it will cost. Also a lot of them will have a bar inside with relatively cheap drinks. The main clientele will be younger european/asian travelers with a few americans mixed in. I stayed at a hostel in Chicago that wasn't very nice, but I had a private room and nothing weird happened.
Not sure if these stories will help or hurt the cause to stay at a hostel........
Once at a hostel in Berlin a guy had a seizure in the middle of the night, a few people woke up, but no one did anything, the guy stopped having the seizure and turned over and went back to sleep.
At another hostel in Barcelona a person sleeping in the same room as me brought back a chick, screwed her in the bathroom. So when I woke up and took my morning piss there was a condom floating in the toilet.