I'll take the second question first. I was surprised that someone would consider an adult reading comic books as unusual (unbecoming?). Perhaps I shouldn't have been so surprised given that I was talking to someone in the software industry, but I digress. In any case I was not trying to "cover up" my reading tastes by using the phrase. I buy and read graphic novels and comics openly; I don't hide my copies of Watchmen, V for Vendetta or Asterix, to name a few. I rank some of them among the best books I have read.
Why then distinguish between comic books and graphic novels? In my mind the distinction is not so much between children's and adults' reading but between the content and presentation. Take for example the 2005 movie Batman Begins. Contrast it with the 1985 vintage Batman. Both are based on the same comic book superhero and narrate similar stories. In spite of their similarities they both are significantly different products. The former is the movie equivalent of a graphic novel while the latter is firmly rooted in the comic book tradition. 'Begins' gained a far wider audience for the reason that it was a very enjoyable movie that happened to be based on a superhero. 'Batman' on the other hand was a movie that played mostly to the crowd already familiar with the story and were in the theater to watch a film adaptation.
The distinction is arguably subjective. I will list below the factors I use to differentiate a comic book from a graphic novel.
- Non-traditional characters. Where traditional comic books have mostly focused on superheroes (with or without superpowers) graphic novels have told the stories of such mold-breaking characters like the Swamp Thing and John Constantine.
- Different treatment of existing characters. This is most vividly illustrated by the success of recent movies like The Dark Knight. These movies were inspired by various graphic novels which told vastly new tales featuring familiar characters, sometimes casting them in an entirely new light.
- Shorter, non-serial story arcs. Comic books typically use recurring characters and story lines that extend across lengthy arcs. Graphic novels are closer to traditional novels in that their story arcs are shorter. Classic examples include Alan Moore's V for Vendetta and Watchmen. This is not always true though; Neil Gaiman's Sandman spans multiple episodes, as do graphic novels featuring Batman.
- Non-uniform presentation. Traditional comic books tend to have a relatively uniform look even when created by different artists. Graphic novels are known for using different artists with pronouncedly different styles in the same volume. The Sandman series is a good example.
- Stylized depiction. This is related to the point mentioned above. Many Graphic novels use fantastically stylized depictions of characters. Frank Miller's Dark Knight, Sin City and 300 come to mind.
- Reading experience comparable to traditional novels. This is quite subjective. I find the experience of reading graphic novels more comparable to reading traditional literature. Take for instance the character Death from Neil Gaiman's Sandman universe. I found that the two spin-off volumes featuring Death read like engrossing short stories.