D20 is the ultimate exercise in adaptability. If you think of D20 as just "D&D 3.5" then you are missing the point entirely. D20 is D&D, Star Wars, Spycraft, Mutants and Mastermines, Call of Cthulhu, True 20, D20 modern, Traveller, and so much more.
You can modify the system and choose from this vast selection of materials to give you inspiration for whatever type of campaign you choose to run. And while D&D complicates things excessively these days, with uberfeats, and uberprestige classes, at its core D20 is just a very simple base mechanic that covers all necessities.
The most purified distilled form of this simple system is exemplified in True 20. Here is D20 stripped of all its "D&Disms", and of anything else that is a holdover to tradition or of unneeded complexity. From that skeleton you can then add and modify to create whatever you like.
In other words, D20 is the game you create. If you just go with what's provided in official D&D rules, then you'll end up with D&D, sure, and at the moment that's becoming a less and less pleasing option. But the point of D20 is that YOU DON'T HAVE TO end up with D&D.
Don't like hit points? Change them; you have a half-dozen options so far for how to change the mechanic, all that have been in official published d20 products.
Don't like classes? change them. True 20 boils everything down to three classes. Mutants and Masterminds doesn't have any class at all. CoC D20 is essentially classless too.
Want to add complexity? That's easy too. Check out D20 Traveller's combat system, among others.
So D20 is basically "one system to rule them all", but not in the sense of GURPS where the understanding of "system" is that you try to jam the same set of rules into any power level or setting context. Rather, its a set of modular rules that you wildly vary and adapt to fit your specific setting or campaign needs.
Its not really one game. Its a design philosophy and a couple of basic conventions.
To say that True 20 and Traveller are the "same game", or that Call of Cthulhu d20 and Spycraft are the "same game" is pretty much absurd. There is more difference between these games than there is between 1st and 2nd edition of WFRP, or than there is between any two White Wolf games.
RPGPundit August 12 2005