The wonderful thing about the OSR movement is that it's a bit like Linux. The core mechanics of D&D are extremely versatile and can be made to do a ton of different things. Everyone in the OSR movement is basically working off the same Kernel, but everyone has customized their own distro to suit their preferences.
In layman's terms: we're all basically using the same engine to power the game, but every variant uses that engine in slightly different ways. And that's awesome. Not just because it allows for ridiculous flexibility, but because the end results are largely compatible. It doesn't matter if the module I'm running was written for AD&D, AD&D2e, Moldvay Basic, White Box, BECMI, OSRIC, Labyrinth Lords, or Swords & Wizardry, it takes very little effort to use it in the framework of Lamentations of the Flame Princess or my own home-brew.
This does raise some interesting questions though. It would be easy to define D&D as the above system, but we also have to take into account 3.x, 4th edition (controversial, I know), and even 5th or D&D next. Each system is as alien to each other as it is the TSR era game. Clearly, changing the way skills work, classes work, special abilities work, spells, etc.. can all be heavily modified without worry.
At what point does the game stop being "D&D?"
The easiest argument would be "If it has the name, it's D&D," but this is intellectually dishonest. First, it precludes Pathfinder, which is effectively a direct iteration of 3.5, and it ignores the fact that the name only has meaning because people associate a certain kind of experience with it. Yes, we could take a 78 Chevette and call it a Mustang, but that doesn't mean people will accept it as a Mustang.
My immediate thought is that D&D requires the following design conceits in order to be "D&D."
- Iconic Six Stats - you could certainly switch this around, but it's such a trademark by now.
- Character creation based on classes - I just can't see a point-based character creation system being accepted as "D&D."
- Earning of experience points - The moment it switched to something like Spiritual Attributes or BW style "beliefs" I imagine people would disqualify it.
- Level-based advancement - This is a constant throughout every incarnation of the game, and something that separated D&D from other fantasy games early on.
- Saving Throws - this is such an odd and counter-intuitive abstraction that it's practically unique to D&D style games.
- Attack roll - math directly tied to class advancement, rather than being an independent skill
What do you think? In your opinion, what are the constants required for a game to feel like a D&D game?