Beedo brought up the idea that "old school" is a play style. Well, that the OSR is a play style, to overly simplify his statement.
I occasionally wonder about this as well, since I play a game that many would consider "new school," as if rules heavy = new school.* Hah, I say. Hah. Hah. But I play with a deliberate tip of the hat to old school games, and the stuff I enjoyed playing since I was a kid.
I think there are two ways to be "old school." One is style, one is literally playing the old stuff.
Old School Style is simply that - you play a game much like it was played back in the old days. For whatever reason - nostalgia, enjoying a different style of play, change of pace, belief in its inherent superiority. Rule sets help - it's easier to play old school if you use an old school game. But it's about style and sensibility. It's that early, fairness means I don't re-roll, cheerfully lethal, no save or undo, go do what you want and I'll rule how it works in play kind of deal. Easier with some game systems but not impossible with most.
Literally Playing Old Games is another part of it. If I dig out my AD&D books and run Dragonlance, am I not playing an old-school game? I'd say yes, even if I'm railroading my players and given them PCs with pages of background and pre-determined actions at certain points in play. That may not be fun (or it might), but it's playing like it's the early 1980s all over again. Those modules came out before at least one of my players was born, and closer to the birth date of D&D than to now. So that's still old school, even if it represents a newer style of play within that old school. AD&D is pretty clearly old-school gaming, but it's not like you couldn't use it for something entirely non-old school in style. And you can use old school rules in a totally non-impartial fashion. Killer DM, anyone? That's pretty damn old school, although rarely fun in my experience.
So I it helps to think of it that way - do I want old school style, yes or no? Do I want actual old school rules, yes or no? You can answer either way, and end up in different places:
yes, yes: OD&D, AD&D, etc.
yes, no: GURPS, Pathfinder, later editions of D&D, etc.
no, yes: Dragonlance, some of the Planescape adventures, etc.
no, no: (Various games)
For me, I think my games are old school in style, even if I cheerfully mix rules ideas from any generation of gaming. I use disadvantages and character background yet I randomly roll treasure in a megadungeon setting and roll damage dice in front of everyone so they can watch their PCs die horribly in the process. I went with yes, no as my options. Whee!
* I've always wonder where folks stick GURPS - is it old school (its central mechanics date to the mid-80s), because of its age, or new school because, uhm, it's still in print? To me, it's still old school at its heart, but the optional and core rules have gotten more and more refined to make it play faster and better. The point values of characters from 4th edition GURPS are different than those in Man-to-Man or 1st edition GURPS, but the numbers mean the same things, and you could use them with only a tiny bit of conversion.