I believe I have reached a conclusion. Thinking back to all games I participated in that I or the DM would consider "old school," as well as games observed and heard about, I have arrived at this conclusion:
Old school gaming is gaming without any agenda beyond simply playing the game. This is not to say that it consists of meandering around the dungeon or wilderness; characters should have goals, but the game itself is not a "story" about the characters, nor is the goal to unravel some plot created by the DM.
Gaming with no agenda...I'm satisfied with that as a working definition of old-school style gaming for now.