[I]n order to exist the series must:
1 – Have an hero. The hero (be it Tintin or Corto Maltese or John Difool) is not a fully developed character, it’s more of a void designed to be filled by the reader with positive things.
2 – A cast of stereotyped characters: the faithful reader knows that this one does this, that one does that. The reader who likes mainstream stuff usually doesn’t want to be surprised (Obelix *always* says that he wants to drink the magic potion; Captain Haddock *always* wants to drink scotch; etc…).
3- A set of stereotyped situations. The plot obeys to a few fixed rules. In adventure comics the thing goes more or less like this: the bad guys attack, the bad guys defeat the good guys, the good guys make a come back and win. The End. In comical comics the hero (or antihero) always commits the same errors, etc…
4 – Adventure follows adventure and the hero and his friends never age. It’s as if nothing happened from story to story (the few exceptions to this rule are far from being perfect).
5 – Psychological depth, what’s that?!

The graphic novel is a strategy to fight the blunt commercialism of the series, it’s the anti-series. Calling a collection of children’s stories (about superheroes, for instance) a “graphic novel” is a co-optation by the sharks, smelling fresh money.

From a blog post by comics critic Domingos Isabelinho.