Follow-up thought, after reading a few others' posts.

To me, what defines "graphic novel" as a medium is the graphics-first nature. It's just a less-stigmatised* alternative term for "comics".

If a story is expressed primarily in text and enhanced with pictures/graphics, then it's not a graphic novel - it's an illustrated novel/novella. This is an equally valid approach, but these are different kinds of thing.

To put it another way: if the story is told exclusively through pictures, and there are no words on the page, it's still a graphic novel. If there are only words and no pictures, it's no longer "graphic", and is simply a novel. Again: equally valid, but a different kind of thing.

By "stigmatised I mean "comics" often has connotations of "Mickey and Goofy", whereas "graphic novel" seems to be more widely accepted to include Maus and V for Vendetta.