I think that "old school" is a completely subjective label. Everyone has different experiences in playing D&D, and it's foolish to claim that there is anything resembling a monolithic expression of the game. From the kids playing tiefling warlocks today to the guys who 34 years ago threw 3d6 in order and chose from fighting man, cleric, and magic-user, the entire point of D&D is that the DM and the players make of the game what they will.
I've been running a number of classic modules in 4e, and am currently working on converting the entire Temple of Elemental Evil. It's been a lot of fun, and it feels old school. On the other hand, I had no compunction about using kenku servants of Iuz in the moathouse, or adding in a summoned demon in the water pool chamber, or replacing Lareth with his agent, a doppelganger who had disguised himself as Burne.
(Lareth and Obmi long since fled for the temple; I did keep Lubash, though. He even kicked in part of a wall as he made his entrance after the green slime at the base of the stairs dropped on the party.)
To me, this is all old school stuff, and my players and I are loving it. I'm sure that, to someone else, messing around with the specifics of the monsters or encounters is some sort of sacrilege to Gary's original work. That doesn't bother me. To me, it doesn't get any more old school than twisting, folding, remolding, and reworking stuff as a DM and gaming group see fit. If anything, mindlessly clinging to classic material as if they are sacred, unalterable texts, goes against everything that AD&D and OD&D have taught me.
I think that, for me, "old school" means:
1. Player choice drives the game.
1a. Really bad player choices lead to TPKs.
1b. Really clever player choices lead to rewards and advantages beyond the norm.
2. There are strains of an almost Lovecraftian incomprehensibility to many gods and demons, a la the chaos temple in Keep on the Borderlands.
3. The forces of evil gather on all sides of the Realms of Man.
That's pretty much it, to me. Rule 1 and its sub-rules are the critical parts of it. Quick, creative thinking is key. Mindlessly attacking is a fool's gambit.