Psion
Adventurer
All these RPGs deserve their OWN system because, like it or not, a system is never “invisible” -- it colours the campaign world itself. The above examples either need a system especially tailored for the world they present or, they once DID have a system of their own but have since prostituted themselves to use the D&D system.
I vehemently disagree, and am given to wonder if you have read or experienced any of the non-D&D d20 games enough to give them an authentic chance.
Are there limits to what a system can credibly do? Yes.
Is that boundary easily and neatly drawn at the boundary of "D&D"? No.
d20 is primarily a cinematic, class-based system. It begins to look a little rougher when you start to do things that classes can't do well (such as supers, since powers aren't learned, it feels a little rough to try to stuff them into classes, which is why M&M sheds the classes and the d20 label at the same time) or that isn't so cinematic. But even then, I have seen some good takes on less than cinematic games, like Traveller.
D&D is big swords and big spells, it is hit points being sloughed off in waves by fire, acid and lightning. It is not a system that lends itself well to science fiction, nor is it a generic system -- only worlds built on the system can truly benefit (ie. Greyhawk, Dragonlance, Unearthed Arcana). A system is like its bones and will shape what the body looks/feels like. If the body happened to come first (eg. Cthulhu, Conan etc.), you can't shove D&D bones into it because they'll never "perfectly" fit. The best you can do is create a unique system especially tailored to the body.
D&D is adapted specifically for D&D, yes.
But d20 is NOT D&D. d20, for example, does not have to use HP.
For example, just TRY to stand up to waves of whithering fire from FGMP-15s in Traveller without armor. You character will be vaporized, even if you are 20th level. And this does not do Science Fiction well? I rather think it does.
It sounds to me like you are basing your judgement of preconceptions.
So, in short, "we who?"