In the middle of an RPGnet thread, some people started arguing about which publications are OD&D and which aren't. It started because the thread originator was talking about how he played OD&D with 2e add-ons, but thought he was playing AD&D at the time. Turns out he was talking about one of the basic sets meant as an introduction to the Rules Cyclopedia. This set some people off.
In the course of the discussion, someone mentioned not caring about tiny little differences between editions. That's what this post, my post, is about: not what defines each published edition of D&D, but what core rules feel like OD&D to me, as opposed to AD&D, BD&D, or WD&D.
To me, six-sided dice for hit dice and weapon damage are OD&D. You can get away with a few minor changes to the latter, like the house rule of rolling two dice and taking the lower for small weapons like daggers or the higher for two-handed weapons; but once you start assigning different die-types to each class for hit dice or breaking up weapons by the amount of damage they do, you've gone pretty far from what I feel is OD&D.
A simpler treatment of ability scores also feels like OD&D to me. No minimum prime requisite scores for the four archetypal classes, smaller bonuses mean less player worrying about not having the best scores.
A d6 roll for skill rolls feels like OD&D. Instead of a list of specific skills you can spend points on, just a flat 1 or 2 in 6 for everyone, with maybe a +1 for someone with a high ability score or appropriate background.
I don't know the post-Holmes basic rules well enough to judge how close to these core rules they are. From what little I've heard, I feel they do drift away from what I feel is OD&D-like.