Remathilis
Legend
Well, since it was my post that brought this up... 
There is a difference between a "game" and a "system". Hear me out.
D&D is a game. Its a specific play experience. If somebody says "I'm playing D&D" I assume certain things; clerics, alignments, pseudo-Tolkien races, color-coded dragons, faux medieval worlds. Unless you specify the game a bit more (I'm running Eberron. I'm running 4e. I'm running Ravenloft) my assumptions are pretty accurate.
The d20 System is a system. I can't say "I'm playing the d20 System" and have you come up with any assumptions beyond grab a d20 and roll high. Are you playing D&D? Conan? Star Wars? d20 Modern? Monte Cook's WoD? There are lots of things I can run using the d20 system, none of which are "D&D."
A lot of people (out of habit from earlier editions) assumed D&D is, in fact, a system. That is, it can gutted to core components and rebuilt to suit different games. However, that doesn't make D&D modular or kitbashable, it just makes the d20 System underneath it remarkably resilient to human tampering. If I'm running a grim, Sword & Sorcery game with Conan-esque trappings, no divine magic, no alignment, limited magic items, no demi-humans, etc, it can easily be said I'm not running D&D anymore, even if the game features rogues, the barbarian class, and xd6 fireballs. I am, though, running the d20 System.
So to me, D&D included a bunch of assumptions about the world, the rules, and the setting around. While you can tweak certain assumptions (such as reworking settings like Eberron or Ravenloft) the more you dismantle the assumptions (from spellcasting to racial mix) the more you drift from the "D&D game" into a "game using the d20 System."
Where that line is differs for everyone, but at some point you cross a line from modifed D&D to d20 Game.

There is a difference between a "game" and a "system". Hear me out.
D&D is a game. Its a specific play experience. If somebody says "I'm playing D&D" I assume certain things; clerics, alignments, pseudo-Tolkien races, color-coded dragons, faux medieval worlds. Unless you specify the game a bit more (I'm running Eberron. I'm running 4e. I'm running Ravenloft) my assumptions are pretty accurate.
The d20 System is a system. I can't say "I'm playing the d20 System" and have you come up with any assumptions beyond grab a d20 and roll high. Are you playing D&D? Conan? Star Wars? d20 Modern? Monte Cook's WoD? There are lots of things I can run using the d20 system, none of which are "D&D."
A lot of people (out of habit from earlier editions) assumed D&D is, in fact, a system. That is, it can gutted to core components and rebuilt to suit different games. However, that doesn't make D&D modular or kitbashable, it just makes the d20 System underneath it remarkably resilient to human tampering. If I'm running a grim, Sword & Sorcery game with Conan-esque trappings, no divine magic, no alignment, limited magic items, no demi-humans, etc, it can easily be said I'm not running D&D anymore, even if the game features rogues, the barbarian class, and xd6 fireballs. I am, though, running the d20 System.
So to me, D&D included a bunch of assumptions about the world, the rules, and the setting around. While you can tweak certain assumptions (such as reworking settings like Eberron or Ravenloft) the more you dismantle the assumptions (from spellcasting to racial mix) the more you drift from the "D&D game" into a "game using the d20 System."
Where that line is differs for everyone, but at some point you cross a line from modifed D&D to d20 Game.