RED cameras use a RAW codec called REDCODE, which records the raw sensor data with lossless compression. Almost every other camera records RGB data that has been through gamma correction, a colour curve, sharpening (ugh..), and a whole lot of other image processing. These other cameras therefore give you very little flexibility in post production for grading and so on.
This gives the illusion that RED cameras have worse colour rendering - because it is not doing much image processing in camera, the un-processed image looks very flat - and it requires some skill to actually expose the image when you're shooting it, and properly process it in post production (this is a professional camera, remember). The amount of crap you see because of errors on set and in processing... But footage from somebody who knows how to shoot it properly is easily on par with the Alexa (which produces lovely skin tones but is very low resolution) and miles better than DSLR cameras with their aliasing and moire that comes from the line skipping and pixel binning that they need to do to get a decent frame rate out of a still camera sensor - not to mention the 8 bit low bitrate RGB codecs they use which are hopelessly lacking as an acquisition format.
And the workflow is sooo hard - I mean, it's such a battle to get RED footage into Premiere for example (File -> Import -> OK). And only pretty much every single grading package, every single non-linear editor and compositing package supports RED footage...