Differences are subtle in the landscapes but the character looks a lot better in DX11. The round filter canister in particular.
It's clear that even now playing Xevius or Gallaga is still a lot of fun despite primitive graphics etc. STALKER's success won't depend on DX11. It'll depend on game play. When you combine good game play with top of the line graphics, well, that's just awesome.
That said I'll happily try the demo of this game on my GTX 260 :)