Wow, 7 whole posts before someone singled out 4e as 'not D&D' - predictable, but still disappointing.
I think you mean "predictable and completely understandable". Particularly in the context of this thread.
It's pretty easy to see why: From OD&D through AD&D2, games published under the D&D trademark shared a core gameplay with such little variation that people would routinely use products across editions without any conversion at all. (The only really significant incompatibility came in the BECMI treatment of race-as-class versus the separate race-and-class treatment of other editions.)
3E largely maintained that core gameplay, although it shuffled the math up and introduced enough new options to character creation to break easy cross-compatibility. (Proof: Take a pre-3E adventure, do a straight conversion, and run it in 3E. It will play identically. Take a 3E adventure, do a straight backwards conversion, and run it with a pre-3E version of D&D. It will also play the same. Fighters play like fighters; wizards play like wizards; etc.)
But then we come to 4E. And here -- despite large similarities -- it's relatively trivial to see that the core gameplay had been completely altered. Fighters don't play like pre-4E fighters. Wizards don't play like pre-4E wizards.
Given this reality, we now come to a thread where somebody says, "So what makes a game 'D&D' instead of something else?" There are two possible answers to this question:
(1) Anything with "Dungeons & Dragons" on the cover is D&D. Everything else isn't. (This answer is trivial, obvious, and incontrovertible in its application. Ergo, no one's going to talk about it. But lemme give it a stab: Do you guys really think of D&D Gamma World as being "D&D"? Do you think of the D&D boardgames as being the same game as the D&D roleplaying games? What about the licensed computer games?)
(2) There is something mechanically distinct which makes D&D different from other games.
And this is where you run into the self-evident problem. If you keep yourself focused to pre-3E D&D, coming up with some set of mechanics which D&D has but other games don't is relatively trivial: You just need to make it specific enough. (Very specific if you want to exclude literal D&D clones, less specific if you don't. Given the criteria you're using, you probably don't.)
Exclude BECMI from your pre-3E sampling and it becomes even easier to find that set of mechanics. Include 3E and you need to stretch it further, but (given the core gameplay) probably still do-able.
But once you had 4E to the mix, you've got a problem. 4E is so fundamentally different in its core gameplay that, if you try to include it, you're going to find it very difficult to stretch a definition that fits just D&D and not a bunch of other fantasy roleplaying games, too.
So, to sum up: You either need to simple define "D&D" as the trademark and nothing more; or you're going to find yourself rampantly and uncontrollably genericizing the term.
Several people have mentioned alignment, which is interesting to me, since there are plenty of groups that long ago either altered or discarded alignment on their own. Are they not playing D&D, by those folks' standards, or is OK to house rule it out, so long as it's at least listed in the core books? What if it's just listed as an optional rule?
Any mechanical distinction you choose to draw has almost certainly been houseruled out of existence by
somebody.
I suspect alignment belongs on the list because it has a pretty notable pop culture presence (at least in geekly circles).
I can envision all three of those working very well in a version of D&D (assuming a new version where the ripples can be handled). But the last one I see as the most modest change, but also the most likely to irritate me.
I think there's a reason why every attempt to add additional attributes to D&D over the past 35 years has failed to take root. Those six stats are a pretty primal identification of the game for many people.
Which, upon reflection, makes sense. After all, what's the first thing most people were asked to do when they first sat down to play D&D? "Roll up your stats." Those six stats are literally people's first impression of the game.