After an entire semester of reading comics, I think the one thing that has surprised me the most is how diverse the medium really is. While after reading Maus in sophomore year I thought I had some idea that graphic novels could be about more than superheroes, in retrospect I realize I didn't have a clue. Even after reading McCloud's Understanding Comics, I was really stricken by just the multitude of ways artists could manipulate art, text, panels, story, and everything else that goes into comics.
True, the same can be said for other media like books or movies. However, the fact that comics can pack so much sensory information into a single page which readers can go back and consider is something which I think can't be said for any other medium. While with books you can go and reread things, there isn't much fun at looking at the same words over and over again.
That's my two cents.