There is still a lot more shared between our games in terms of rules and implied game physics than there are differences; because the mechanics, powers and class abilities have some considerable weight of genre expectation built into them.
I'm not really trying to argue with you CJ, just expanding on the experiential context of my previous post.
Sure. There has be a certain amount of shared stuff, or we couldn't even talk about as "D&D". The trick with D&D is that it's a giant Venn diagram where practially nothing is in the center bit that all the circles touch, even though there is much, more more that a lot of circles touch.
But even amongst those various worlds there were, barring special abilities or DM intervention, commonalities. As an example, one such commonality is that a greatsword will always, on average, do more damage than a dagger. That may be pure mechanics but it directly correlates to and influences the fiction of a D&D world.
OD&D had all weapons doing the same damage, 1d6. So this is a great example of exactly what I said above--it's mostly common--so much that a lot of us think of it as common, but then we find out when we look closer that it isn't exactly so. It's also a good example because it shows how the things can grow into the awareness of D&D and then become the perceived D&D way of doing it.
(Responding to both quotes) My larger point, however, is that we are saying across versions and campaigns here is
even more true across individual tables. Sure, let's pick and example that should favor this idea of common D&D. Take the genre expectation of "fighter in mail, with longsword and shield, bow on his back, delving into the dungeon to fight goblins." Even with the changes in all the versions, we can all discuss that concept, right?
Yet I guarantee that the play experience of that fighter concept has significant differences at my table compared to other tables. Significant differences compared to your respective tables? Maybe, maybe not. I don't know what goes on at your tables well enough to say. Significant compared to the whole range of tables? Darn straight. I don't need to know what goes on at every table to say that confidently--merely that some people have conveyed what goes on at their table sufficiently well that I know it is different than what happens at mine.
You can draw a big circle around
most of that D&D Shared Experience Venn diagram, and the stuff inside the big circle is this thing we call "common". (The better job you do of putting the line in the right place, the more useful it will be.) Stuff inside it are things that people know about, even if they don't use it themselves. I'm not big on psionics, for example, but I'm aware of psionics sitting there where a bunch of other peoples' experiences overlap.
Then you get things that are further afield, sitting on the edge of that big circle, like Birthright dominion bits, Spelljammer-specific concepts, the esoteric Planescape knowledge, actual experience using AD&D grapple rules in play

, 3E gestalt options, the duelist and half-ogre, 4E explicit narrative concepts, etc. These are all at least overlapping the circle significantly, but they are in a minority of individual Venn circles.
Then you get things that are outside the circle entirely, such as Runequest skill manipulation or GURPS character generation, for two examples out of thousands.
All of this conspires to make the "mostly common" stuff seem like it is "exactly common". But it's the Princess Bride difference between "mostly dead" and "dead dead"--though reversed in this example.
