As graphic novels are by their very definition a blend of writing and illustration, it goes without saying that they will achieve a level of descriptiveness that a traditional novel will fall short of. Art and expression are, rightfully, in a constant state of stylistic evolution. You can even find works that blend the traditional novel with the graphic novel, such as Mark Z. Danielewski's House of Leaves. As mentioned in another response, graphic novels allow for the easy conveyance of surreal and abstract ideas. Art forms like illustration and graphic design lend themselves readily to such forms of expression.
However, one aspect that you could express about traditional novels in words is that they are a more collaborative experience between author and audience. Imagery is key in the traditional novel simply because it allows the reader to picture the images being described. Readers must create the physical and metaphysical landscapes of the work for themselves. This creates an experience wherein the world of the novel can become more personalized to the reader's unique experience. This stands in contrast to a graphic novel. In those, the physical detail of what the world looks like is already explained through the work of the illustrator.
The differences you have listed between conventional and graphic novels are valid and quite thorough. However, I would like to add one or two points that would enhance the distinctions you have outlined.
If one looks at a specific graphic novel such as Art Spiegelman's Maus, one can see immediately a striking effect that is impossible in the conventional format, despite the fact that in the twentieth century and later, written prose, through stream-of-consciousness and other techniques, has already been able to achieve surreal and dreamlike effects diverging from photographic reality. In Maus,Spiegelman casts people as animals: specifically as mice, cats, and pigs. Given the use of anthropomorphism (the ascription of human characteristics to nonhumans) in cartoons and children's literature, one would not necessarily think that Spiegelman's technique is necessarily revolutionary or novel. However, in Maus, he is using this method to symbolize the dynamic among the different groups during historical events: World War II and the Holocaust. A conventional novel, regardless of how imaginative and surreal it might be, would never achieve the striking effect of Spiegelman's images upon the reader or effectively utilize the grim symbolism inherent in his use of animal characters as humans. History is thus given a new kind of life in which the immense tragedy of the Holocaust strikes the reader with an immediacy that neither the conventional novel nor film is capable of.
It sounds like you have a great understanding of the structural differences between graphic and traditional novels, including uses and effects of such differences for the author's purposes. Perhaps the next item of comparison should be how these two types of texts affect readers differently.
While reading a traditional novel, we experience the flow of "natural reading," meaning that our brains register whole sets of words simultaneously, rather than focusing on individual words (which can happen in short dialogue bubbles of graphic texts). Therefore, we cover more text in the same amount of time we would spend on a graphic novel, which generally has shorter groupings of text. As we read a traditional novel, our brain registers the word groupings using various area of the brain (touch, vision, emotion, etc.) To fully make meaning of the words, the brain then associates them with our own real-life experiences. Of course, all this happens within milliseconds as our eyes scan across the page. However, this also means that we don't take much time to consider any one idea (generally required for retention of data) unless we choose to stop and re-read or take notes.
Due to the verbal-visual blend of a graphic novel, there is generally less text to be read on each page, and the flow of reading is interrupted more often as our eyes scan to the pictures. However, studies show that this multi-modal method of reading is actually more effective for retention of data. Rather than relying only on textual evidence, our brains must also decipher visual and spacial evidence on the page. As a result, our neuron activity is increased as we read and even afterwards, as we remember the story and accompanying graphic images. For this very reason, foreign language learners not only have an easier time reading a graphic novel in the new language, but it also helps them acquire and retain the language faster. The same is true for anyone wanting to acquire new data.
One study done at Ankara College Foundation Private High School compared similar-level students’ comprehension of Shakespeare’s Macbeth. Not surprisingly, the group using a graphic novel approach scored significantly higher on the posttest than did those reading the traditional play format. The days of believing that comic books are just for young children are past. We live in a multi-modal, digital world, and whether we simply want to enjoy a colorful story or acquire some new knowledge, choosing a graphic text is not only fun but an effective brain activity.