For me Old-School means: Random, random, random. Everything is about the randomness, using the dice at each and every opportunity. Everything has to have a chance at failing, even jumping the small ditch in town. It is more about what story the dice tell as opposed to the characters.
Also, it is about things being a little more simple. (Or, maybe, that was just my young mind.) No uber-detailed, intricate plots, just bad things out there in need of some killin' - and their stuff in need of being taken, know what I mean?