You truly have not seen a Marx Bros. movie, and where have you listened to them? They went from vaudeville straight to movies, then Groucho went to radio for a while, and then he went to television. And their jokes were very natural, some were obviously set up, but that's because they were always playing off not so great actors and comedians, but when Groucho and Chico face off it's always hilarious, and natural. The plot of the EPA places itself because the 80's had a very environmentally conscious attitude. Today the bad guy would not be the EPA. As for the plot being genius, well then you don't even know your Ghostbuster mythos, the fact that they are Ghostbusters is taken from various incarnations of comedians playing Janitors of the paranormal, like the Bowery Boys, or Abbot and Costello, even Mickey Mouse, Goofy, & Donald played a team of Ghost Catchers in the 30's or 40's for a cartoon. And the rest of the plot comes from just creating some sort of central conflict for a climax, other than that, not as important.