This is why. (click the picture to enlarge it)
And this is from an HDTV (1920x1080) downconversion. The original file is at 2k resolution (and it can shoot at up to 4k resolution).
The only thing I've noticed is that medium to light pastel blues tend to pop out unnaturally, but some of that can be minimized in post-production color correction.
This short was shot using a set of fairly expensive Zeiss prime lenses. While the RED-branded prime lenses appear to work quite well (and I would consider shooting with them for some projects), good quality film lenses like the Zeiss and Cooke primes handle the shape and depth of the image in different ways. They aren't necessarily as "accurate" as the RED lenses, but they give a more classic film "feel", which personally I think helps to sell it to those filmmakers who are going to use it in place of film, since their end product will conform more to what audiences expect to see (at least subconciously). As an exercise, note how the focus falls off in the image above, then go look at this Red Prime lens test. A more extreme example is if you look at films shot using anamorphic (film) lenses, which can really distort the image around the edges, yet still remain pleasing to the eye (check out Road Warrior for a good example). Car commercials in the last few years have taken to computer-generating the distinctive lens flares from anamorphic lenses because they like them so much.
I'll try to get up a clip of the short this still is from in the near future so I can make a better comparison.