It seems bizarre to draw a distinction based on how a book may have been previously printed. If I’m holding, say, a 160-page squarebound book, containing one whole story, and I don’t happen to know whether it was also published as a serial, what do I call it?
I voted “Format,” but it’s really a combination of subject matter and marketing. Depending on how that’s handled, any comic book could conceivably be a graphic novel, and vice-versa.
I don’t really consider there to be a difference, since most of the times the person trying to make the distinction is doing so in the “comics are for kids, graphic novels are for adults” way that typoink mentioned. Especially since many works constantly touted as “graphic novels” are just trade paperback collections of previously individual released issues.
I kinda wish that “comic book” wasn’t the term for the whole medium, since most comic books aren’t actually comical. But that ship has sailed a looooong time ago.
Even the monthly titles from the major houses (Marvel and DC, at a minimum) are now printed on high-quality (if thin), glossy paper. Compared to the cruddy newsprint-style paper from the 1970s and 1980s, when I first started reading, it’s a dramatic improvement.
Fortunately, humor in comics is still alive and kicking! Fiction Squad was a delight, following in much the same paths as Fables, which, while more serious, had no shortage of whimsy. The new run of Doctor Strange is quite comical (most evident in issue #1, slightly less in subsequent issues.)
Even the usually dour comics take delight in comic asides. Current X-Men comics, for instance, while bleating about the standard “Mutant, mutant, angst, angst,” will have charming side-dialogue, most often involving Bobby Drake, Iceman.
(Then there’s Deadpool, which is pure comedy… Nasty comedy, and I frankly hate it, but it is comedy.)
(Am I gonna get disembowled for that?)