Commenting specifically on Watchmen, I love the cinema-like visual narratives that run throughout the novel.
For example, there's the novel's opening image of (couldn't resist using an actual smiley-face visual since it's right here on the bar above my box of text!). I love how the significance of the novel's cover becomes apparent only as the "camera" pulls back from the extreme close-up to reveal the larger context of the entire "scene" through an extreme high angle shot.
Sometimes Watchmen's visual elements will be a recurring "joke," like the Gunga Diner, which actually is a reference to Rudyard Kipling's poem, which in turn offers interesting commentary on the novel's theme. Other times the visuals tell their own stories that run in parallel with and complement the main storyline.
I really like the interplay between image and text that the graphic novel format offers. Although sometimes graphic novels are criticized for "dumbing down" the reading experience, I think the format actually can enrich the reading experience and make it far more complex, at least for readers who fully engage with the material.