One thing I have come to understand during my research, is that there is no real categorization system for Art directions in video gaming. Strictly speaking, this seems to be a good thing as it is not really what games are sold on and being able to pigeon-hole art in such a way would suggests either saturation or lack of variety. I have found, after asking a number of my friends that the default way people seem to refer to games is with either some of the blanket terms I have used previously (2D, 3D, Realistic, Cel shaded, Pixel ECT) or by comparing them to other, visually similar games.
Whilst I do not think it is possible to properly describe art styles in 1-3 chunks in a way that game genres are described, I believe small sentences would suffice in this area. To test this theory out, I picked a number of my favourite art styles in video games off of the top of my head and attempted to describe them without referring to other, similar games.
A grim underwater metropolis with heavy Art Deco stylings
Deus ex : Human Revolution
Futuristic Cyberpunk with sleek modern stylings and a strong yellow focus colour
Vibrant, Colourfull and cartoonish Art design with a mix of qwirky characters and a trendy, hip hop style.
The wolf among us
Cell shaded noir with heavy comic book stylings.
Black and white with a heavy emphasis on tonal shifts and blood as emphasis
Guilty Gear XRD
Cell shaded anime stylings with traditional animation techniques for dynamic animations
Shadow of the colossus
Epic fantasy art with a grandiose sense of scale
Heavy gothic influences with complex religious architecture (Grand cathedrals and the like)
Describing a game using a sentence is clearly possible, but as far as offering a quick impression of a games visual style, simply showing a picture would be far easier. Showing gameplay style is far harder with pictures than visual style, so genre descriptors work well for gameplay but images are a far better method. In truth, upon finishing this, I have realised this is a problem that comes from semantics rather than any real issues or discoveries, which is unfortunate.