I think even on the PC, where HL2 looks significantly better, it's the textures, not the tech, that is allowing the game to age gracefully. Episode 1 (and I am assuming 2) have more pretty graphical tricks up their sleeve. HL2 just has impeccable visual design.
Portal has really good, consistent visual design, too, but it's hurt more by the lack of AA because it's more of an exercise in visual minimalism, which means you are paying more attention to the edges of things, which due to the lack of AA are the worst looking part of the package.