Basically the way it responds to light doesn't match the response of certain popular 35mm stocks. It has a high dynamic range, but that alone doesn't mean it will behave like film. It just means its video with high dynamic range.
The reason why the industry has such a boner for the Arri ALEXA (despite not being up to par with RED's specs) is because it apparently handles like film. Experienced DPs will often initially meter a scene, and as the camera/light/subject moves around (all things that effect exposure), they will use their eye rather than re-metering before every single take. They are able to do this because they know how film is going to react. The ALEXA caters to this workflow, whereas other digital cameras respond to light in their own ways, and the DP has to check before each take.
Besides, there's so much more to a digital camera's "look" than specifications; don't let gearwhores fool you into thinking otherwise. If the RED and ALEXA had identical specs, they'd still put out completely different images.