As far as I know, people first started talking about graphic novels when Watchmen first got collected into a single volume, presumably because DC realised that it was too good to let it go out of print and because there was a lot of money to be made from selling it in conventional bookshops. I love Watchmen as much as the next person, and the comics medium's desire to be taken seriously and accorded the literary merit it deserves is admirable, but appropriating terminology from other media isn't the way to go about it.
To me, 'graphic novel' just comes across as a bit condescending, as if comics aren't good enough in themselves and so have to borrow another, more respectable medium's name before they can be treated with the same respect as traditional literature. Admittedly, there are many cases where it is a fairly appropriate description, but even in those cases I simply don't see what's wrong with calling them comics, because that is fundamentally what they are.
When a comic is conceived of, written and published as a single, reasonably long volume, in the same way that a novel is, then it is fair enough to refer to it as a graphic novel, even if my aforementioned issues with the expression still stand. For instance, Blue is the Warmest Colour is a graphic novel, and Hellblazer: All His Engines is a graphic novel. There are certainly advantages to be had in writing and publishing your work in a single, novel-length work rather than serialised in periodicals, and there have been no shortage of great books in that format. The authors arguably have greater creative control, aren't constrained by page limits, and don't have to worry about deadlines. I've no issue with the format, I just think the terminology is a bit pretentious.
The real problem for me is that, nine times out of ten, when people say 'graphic novel' they mean 'trade paperback,' and while the terms are almost always used interchangeably, they ought to mean completely different things. Most of the time when people refer to graphic novels, they're talking about collected editions of monthly comics, in which several issues have been put in one book for convenience, ease of reading, and so that it can be sold in traditional bookshops.
Again, I've no issue with the trade paperback format. I don't buy single issues because the trade is usually cheaper and doesn't have adverts in it, and the fact that comics are being collected and preserved in this manner is a wonderful thing for the medium. Most writers tend to write for the trade these days anyway; it's pretty rare that you'll come across a single issue that actually works as a standalone story rather than as a chapter in a bigger, ongoing narrative. But this is where we come to the real crux of my problem with graphic novels.
Putting six issues of a monthly comic into one book doesn't make a novel any more than putting six episodes of a TV show on one disc makes a film.
The 12 issues of Watchmen may tell a single story that was clearly planned from the beginning as such, but that doesn't make it a novel. The 8 episodes of True Detective's first season also tell a single story that was clearly planned from the beginning as such, but no one in their right mind would refer to it as a film.
Like I said, it's a symptom of the medium's inferiority complex. It's a pretty old medium at this point, but given that its development as an art form was arguably set back at least 20 years by the Comics Code Authority, comics as a medium still has a reputation as being for kids, as unfair and undeserved as that stigma might be. It's simply a case of a relatively young medium borrowing terminology from an older, more respected medium to describe something that is uniquely its own, and doing itself a disservice in the process. It's implying that the comics medium is somehow inferior to the novel medium, which just isn't true. Neither is superior to the other, they're just different.
So there we are. While there are cases where 'graphic novel' is a pretty accurate description of a comic, for the most part the term is condescending, not applicable to the format, and a statement that comics aren't as good as prose.
And that's just a bit rubbish, isn't it?