I agree that digital cameras lose that sort of suspension of disbelief we get looking at images shot on film, but honestly the reason why the Hobbit one looks fake while the Fellowship one looks real is because the Hobbit one is just Martin Freeman shot on a green screen and then composited onto a CGI environment. Elijah Wood is standing on an actual set. Granted, there are matte paintings/miniatures and probably minor CGI tweaks for the set extensions, but that's all background stuff. The part he's standing on, the part he's interacting with, is 100% real. And that's what makes our brain williing to believe that the rest of the stuff in the background is just as authentic.