Hussar
Legend
I guess I'm with @Thomas Shey on this: house rules don't stop D&D from being D&D. I mean, think of it this way a moment: what percentage of DMs and players actually even know all the rules?
Suppose some table of friends (casual players and DM, all of them) plays what they call D&D, but they have no idea what the flanking rule is or how it's used. Further suppose the players on their own come up with a flanking action in some pitched combat and ask the DM for her judgment on whether they can get some kind of attack bonus for it, and she says, "Sure. I'll give you bonus x under conditions y." Now further further suppose the bonus she gives them is not the one in the book. Are they not still playing D&D? I certainly would say they are.
Yeah, being "not D&D" is probably too strong a term. But, the basic point about calling it your own game is a decent one. Because we see it all the time when two posters are completely talking past each other because their play styles and play experiences are so different from each other's that they really, really aren't playing the same game.
One poster, @Lanefan, that I constantly stand in awe of runs these decades long campaigns. I think my longest, ever, campaign was like a year and a half, maybe two years? The vast majority have been one year or less. Which means that we constantly have very different views on gaming because, frankly, neither of us plays the same game. What works for me and what works for him are completely different because every single thing at our tables is based on entirely different priorities.
Which doesn't make either of us right or wrong. That's always the mistake in these conversations. Trying to determine a "right" answer. At best, you can get a "Well this is the right answer for me at this point in time" answer. Which might be the entirely wrong answer six months from now and would have been the wrong answer six months ago.