One of my greatest complaints about graphic novels is that very few of them are novels at all, by which I don't mean that it is ridiculous to call a memoir like Alison Bechdel's Fun Home a novel, though clearly it is, but that the amount of narrative material in most graphic works barely amounts to a novella. Black Hole may have taken Charles Burns ten years to put together, but I read it in a little more than an hour. It's hard for a work that demands so little of a reader's time to develop the breadth and heft that I associate with novels (or novel-like works of non-fiction).I think Abigail's right about this, but I'm not sure that's the right frame of reference. I think graphic novels, in terms of the amount of "content" (however defined) they contain and the amount of time they take to absorb are comparable to movies. Movies, too, take "a little more than an hour" per go-through (1.5 - 2, usually, sometimes longer). And movies generally also have about a novella's worth of content to them (in my experience, novels made into movies need to be cut, short stories expanded; novellas work about perfectly).
Note that movies, too -- like graphic novels -- take an inordinate amount of time to produce: usually more people in fewer time, rather than fewer people in more, but still a great number of hours of human labor go into them.
I don't think this has much to do with the potential of the various mediums, nor with their youth/age or anything; I think it's mostly a matter of how human beings absorb visual (or in the case of movies visual/audio) versus linguistic information.
There are, of course, some graphic novels that take a lot longer to read (as, in fact, Abigail has noted in the past on her blog -- it'd be interesting maybe to compile a list...), just as there are long films, or films that flatly require multiple viewings for comprehension (and are thus effectively longer), just as their are novels that are short and rip quickly by. But these are the exceptions.
For graphic novels, incidentally, an additional limiting factor may be financial: it costs more to print illustrated pages than prose, and so graphic novels can't be as long as, say, a Dickens novel. (Or, if they are, they are serialized -- as was Dickens, for that matter.) Not to mention the basic financial issue of paying for the work to create the thing (whether lots of people for a year or two or a few people for many years). Thus lengthier comics narratives tend to be serials -- again comparable to movies, where the longer works are, in fact, TV shows (which in better cases work as long, serialized movies) -- where the financial burdens can be spread over time, interest can be gauged, etc.
As far as Abigail's complaint that "it is ridiculous to call a memoir like Alison Bechdel's Fun Home a novel", she would perfectly right if it were prose.* But, as I've noted before, this is simply the way the term "graphic novel" has been used since its introduction: it's never just meant the comics equivalents of what in prose would be called novels (literally since the term's introduction: the first widely-publicized work to be called a "graphic novel" (Will Eisner's A Contract With God) was a collection of shorter stories). A "graphic novel" is, generally, a book-length, sophisticated* comic of any variety, whether a single work or a collection, and whether fiction or non-fiction.
To repeat the point I made in my earlier rant on this: "graphic novel" is an odd and imperfect term, I admit, since it sounds -- to those unfamiliar with the medium -- like it refers to a type of novel. But it's the term we have -- the "wrong and only name for it" (to borrow a phrase from David Hartwell in referring to another publishing category ill-served by reviewers). It's now an official category in many bookstores. There are magazines and web sites and college classes on the form. It's what these things are called.
(And again, I think this is a common linguistic phenomenon -- that is, that a compound term will include items that won't be within the realm of the root term. I don't know the name for this, though, if there is one. Is there a linguistic in the house?)
Finally, a few stabs at the list. What is being listed are graphic novels that take a long time to read -- ones that have the heft of a novel in terms of time it takes to read them (putting aside the issue of artistic merit or lack thereof). This is easy to do if you consider a serial as a unit; but let's restrict the list to items available in a single volume. Books that, in Art Spiegleman;s words (in describing his ambitions for Maus) are comics that you need a bookmark to read. Off the top of my head: Maus, natch; Alan Moore & Eddie Campbell's From Hell; Alan Moore & Dave Gibbons's Watchmen; Jeff Smith's Bone; many of the various Love and Rocket books by los bros. Hernandez (certainly the really big hardcover ones published a few years ago); a few volumes at least of Dave Sim's Cerebus; Alex Robinson's Box Office Poison; Chris Ware's Jimmy Corrigan; and both issues of Shane Simmons's Longshot (which is pretty impressive for 24 page comics). A few of these are arguable; but there are a lot more, too. Leave other suggestions in comments.
* Although the mistake is so common among my students that I wonder if the word "novel" is shifting into simply a pretentious word for "book" -- or, perhaps, a word meaning any book-length work, as long as it is a single piece (and not a collection of shorter ones). If so, it's a trend I'd resist -- there'd be no word left for "novel", which is something worthy of a term -- but in matters of linguistic change resistance is, most often, futile.