I think the term “literature” is often misused and/or misunderstood. My bachelor’s degree is in English Lit and I’m come to understand “literary” as a term applied to a work with complex themes, characterization and message, presented in a deliberate (sometimes “artistic”) style. Plot is usually, though not always, secondary. The Adventures of Sherlock Holmes, though fun, is not literature; Frankenstein is. Conan the Barbarian is not literature, A Clockwork Orange is. Paradise Lost is definitely literature, Twilight is not. I can go on and on.
Now, according to my criteria, there are graphic novels that can be considered literature. Watchmen, though I’m not crazy about it, is definitely literature. It is perhaps the most painstakingly crafted comic ever. The character design, the panel layout, the symbolism in the artwork, the themes and even the color palette were all very deliberately designed to make a complex, thoughtful work. The same can be said for the phenomenal Maus (which won a Pulitzer Prize). Reading graphic novels like these require a sharp eye and a level of discernment similar to what a reader would put into understanding a literary work.
Comics get a bad rap from people who still think of comics the way they were decades ago; they think of comics as a variation on illustrated children’s books with slightly more pictures and less words. Comics have grown up, and few people have noticed. This outdated view of comics, ironically enough, is the fault of the two major comic publishers, Marvel and DC. Based on the success of the “graphic novels” of the mid-to-late 1980’s, with their adult themes and literary merit, the big two wanted to cash in and started packaging their monthlies in a thicker paperback and calling them graphic novels (the term should have been reserved for the more serious stuff). They diluted the market and the casual consumer can’t tell the difference between the pulp and the stuff worthy of merit. Don’t get me wrong: I’m a huge fan of the pulpy monthly books like Spider-man and Superman, but they are definitely not literature. The OP mentioned The League of Extraordinary Gentlemen as an inspiration for this question, but I wouldn’t consider the graphic novel literature. A fun read, but missing that gene se qua of true graphic novel literature. Here’s a short list of some works in the comic medium I consider “literature” (and I’m excluding lots of books that are great fun, artistic, creative and entertaining, but aren’t “literary”):
Watchmen
Maus
V for Vendetta
The Sandman
Pride of Baghdad
Transmetropolitan
Locke & Key
The Invisibles
The Path
Pax Romana
The Walking Dead (this one’s on the edge between genre pulp and literary)
Y: The Last Man (like the above, I almost didn’t include it on this list)
I heartily encourage anyone who has doubts about graphic novels being considered “literature” to look over the books on the above list (particularly the first five or six) and try to explain to me how these works don’t accomplish the same thing traditional literature does.