What Is the difference between traditional extended passage of dialogue and modern extended passage of dialogue?
I am not sure if this is exactly what you are looking for, but I'll do my best.
In looking into the many types of dialogue, each serves a purpose to provide information to the reader, to drive the plot, and to do so through dialogue rather than simply with the narrative.
In reading over passages of older books, I would surmise that the traditional extended passage of dialogue may have been used more to develop characters, especially the connection between two particular characters and to, as mentioned, drive the plot forward.
However, with the modern extended dialogue, my perception is that dialogue has become more sophisticated: serving to act persuasively, to find information, to develop an argument, or even carry out negotiation. Extended dialogue today seems to have a much more specific job than to simply develop characterization or move the plot along.
For this reason, I believe that truly succinct and well-written dialogue by today's authors requires the reader to pay particularly close attention rather than skimming through. Too much of importance is being conveyed through the conversation of characters, and should not be casually brushed aside. Whereas we have in the past looked closely at the narrative for deeper meaning, I believe that writing today uses all the tools available, including dialogue, especially as authors delve more deeply into a better understood psyche to find motivation for the actions of a story's characters.