A lot of it comes down to how the art handles emotion. Static sprites have to convey a huge range of feelings through subtle shifts—a slight change in eyebrow angle, a different mouth sprite, a shift in lighting. The iconic moments are often the 'CGs' (computer graphics), the full-screen illustrations for key scenes. In 'Planetarian,' the image of the robot girl Yumemi looking up at the projected stars in the ruined planetarium is burned into my memory.
It's a perfect composition: her hopeful expression, the ethereal light of the projector cutting through the decay. That single image encapsulates the entire theme of the story. Iconic art direction knows how to compose these payoff moments so they feel earned and emotionally devastating or uplifting, acting as the visual climax to hours of reading.