I believe the visual element to be most important (eg. a movie shot on film will always be more desirable than being shot on home video unless, of course, relevant to the story/ approach), but that is not the only criteria by which I judge a film. A perfect film for me will have both style and substance; however, if a film has a lot of substance than the need for style is not as important, and if the film does not have much substance, then it will need a lot of style.
Avatar on paper has little substance, but the style and the spectacle of the film, unlike anything I had seen before (the dragon ride, the fluorescent scenes, the original creature designs, the final battle etc) made up for its shortcomings in the substance department to make it a great film (though obviously not perfect).
Avatar is a film designed to create spectacle and to criticise it wholly on plot and characterisation is shortsighted. As an action film, plot and character are not as important as the way the film is told, and its setpieces. Obviously strong plot and characters are preferable, but if not truly awful, then strong style can overcome this weakness.
As for The Man From Earth, I have not seen the film, but I am sure that it does not try to be a spectacle in the same way Avatar does; it would therefore be stupid for me to criticise it wholly based on its lack of spectacle. To make up for this, I am sure that it has an engaging story and original and interesting plot, and therefore I will not miss the lack of spectacle.