By that definition my campaign is medieval fantasy. But that is a VERY broad definition. A LOT broader than I'd interpreted it.
The other way to approach the whole "D&D core" topic is from the other direction.
What other games would be "D&D core" by these guidelines? How many D20 variants? "Blue Rose" would be. Mutants and Masterminds with the Fantasy RPG module would arguably be (classes is a little problematic).
But I'd consider neither of them to be D&D. They "feel" different to me in hard to articulate ways.
Is a game of pirates using Pathfinder rules D&D? I think that Monte is saying no. I think that I'd say yes.
I'd agree with you on
Blue Rose or
Mutants and Masterminds not being D&D. Similarly, a
d20 Past campaign that uses the
d20 Modern rules may be d20, but it's not D&D.
However, I think it's pretty clear that
Pathfinder, like
Castles & Crusades,
Swords & Wizardry, or even
Hackmaster, is a D&D clone. So Pathfinder and all the rest count as D&D - "the core" is largely intact.
The very fact that we can agree that certain games which aren't actually labelled as D&D (
Pathfinder, Castles & Crusades, Swords & Wizardry) should count, and that certain games which similarly share the mechanics (
Spycraft, Star Wars Saga Edition, and
d20 Modern, for example) clearly means that there IS a "core D&D" that exists. Part of it is the setting, but part of it is tied to the rules as well (even if they used the Greyhawk or FR setting, nobody would claim
GURPS Fantasy or
Warhammer Fantasy Roleplay are D&D).
However, the heart and soul of D&D includes its ability to be houseruled. As an example, what if I go back to 3.5, but chuck the PHB classes, in favor of the Spellcaster/Warrior/Expert variant classes from
Unearthed Arcana? I think most of us would agree that my game is still D&D (albeit a little bit...tweaked). I could add in a couple classes (even all?) from the
Arcana Evolved variant PHB, and most would still say they were playing D&D.
Similarly, if I'm playing a 3.5 game set in Sanctuary (using Green Ronin's
Thieves' World Player's Manual), or Fantasy Flight's
Midnight setting, the game is still (by almost everyone's definition) D&D, even though it uses (in both cases) a modified magic system and/or modifies several classes. Similarly, a game set in
Dark Sun or
Dragonlance doesn't cease to be D&D because it cuts out a couple classes. So clearly, the game can tolerate a substantial amount of deviation from "the core," although everyone may have a different tolerance for how much deviation they permit.
Back to the point, I'd definitely say your hypothetical pirate game using
Pathfinder IS D&D. Because, if you were using the D&D ruleset (a 3e game set in Freeport, for example), there wouldn't be a question. And I think Monte would say likewise, since I'm pretty sure Monte would agree that
Pathfinder counts as a D&D clone. Hell, the introduction he wrote for it says as much!
How far can a game deviate from "the core" before it no longer counts? That's a very good question, and it may be in large part subjective, but there's no doubt in my mind that a "core D&D" does exist. And I think Monte has a pretty good handle on what most people want to see in it.
Yes, you can eliminate some aspects of the core and/or replace them with variant rules, and people would still call it D&D, but you wouldn't really expect to see a game labeled
Dungeons & Dragons that didn't have them. An example of this is 4e's replace system for alignment and its wholesale replacement of the classic spell system. As much as some gamers like those things, it was just "too much" for many people.