As far as artistic game development goes, there appear to be 2 schools of thought: Expressionistic aesthetics allow the designers to better portray emotional states, or heighten the absurdity/drama of a moment; photorealism lets players immerse themselves more readily into a world that might be unfamiliar to them, and generates a stronger feeling of verisimilitude. Of course, your mileage may vary, but in general, these modes of thought hold.
When it comes to MMOs, where an overarching story frequently takes a backseat to game mechanics, which style provides a better experience? Would WoW benefit from an overhaul of its graphics, making things more realistic-looking? Would EVE Online become a completely different game if everything were cel-shaded and anime-like?