You cannot classify games based upon the dice they use, just as you cannot classify games based upon the fact that some of the material is available for anyone to use. Hasbro/WotC defined a "d20 System" game as one that required the d20 Players Handbook to use, meaning it uses those basic mechanics without alteration. Games using only the OGL do not have that restriction. That freedom is what allows Castles & Crusades to take mechanics such as feats, skills, and attacks of opportunity, and chuck them into the dustbin.
Okay, I started this mess, and now I'm throwing my fat into the fire again. Here's what I think now (if you care):
The basic core of the game, minus the Siege Engine, comes from AD&D 1e. Side by side, these games are recognizably similar. Now, everyone here knows that this was intentional. C&C was intended as a stylistic throwback to AD&D 1e.
All of the TSR products that are part of the D&D family are the original D20 System. WotC's legal nomenclature notwithstanding, even 3e is a recognizable outgrowth from what came before (especially if you have the 2e "Combat & Tactics" book, from which most of the 3e combat stuff was taken).
To that end, I'll still contend that C&C is a D20 System game, in that it uses material intentionally taken from D&D, instead of creating its own system from the ground up. In this case, it was done on purpose as a reaction against 3e (citation needed; my understanding is that they were trying to make a simplified version of the game that hearkened back to the previous era
). It's not bad-mouthing the game to note that it's derived from an earlier game, using material that they took from another game that they were legally allowed to use, and then added some ideas of their own into the mix. None of this is an attack.
However, by legal definition, if it went into a court, it appears is that the legal name for it is an OGL game. I admit, I tend to use the terms OGL and D20 somewhat interchangeably because, well, I'm not really hip to all the details of what exactly makes one what it is (this thread has helped a lot with that, thank you all).
So, while I still contend that it is a variant of D&D, specifically the first edition, I also now understand that it is legally an OGL game, and that this is not necessarily the same as D20 game. I also don't consider calling it a D20 game a pejorative, either.
My biggest sin was calling it an "iteration." By definition, I think that's the wrong word. I stand by "variant," however.
The term D20 System game, while possibly technically correct, is not legally correct. It implies certain legal requirements that are NOT a part of C&C. So, while it may have its basis in D20, it is NOT a D20 game in the legal sense of the term. I think it is important to make this distinction so that lawyers cannot take away our game.
It is, legally and by definition, an OGL game, however. This does mean it was derived from rules set forth by WotC, but can be considered it's own game under that definition. Whether or not it's actually derivative (e.g. uses spell names like Magic Missile), the OGL still permits it to be published as its own game, which cannot be revoked. Again, this is important, to all of us.
So, what have I learned? I have learned that the correct thing to say is, "Castles & Crusades is the best OGL variant I have ever seen." That is true and legally correct.