If the reader suspends the narrative of Poe from any connection to moral reality, and judges it merely upon its construction, then the positive value in "The Cask of Amontillado" is that it is a superb story that meets all the requirements set by Poe.  For, it has, as Poe demands, a singleness of purpose that it, indeed, achieves.  Through all of the arabesques, as Poe terms the turning and twisting of the action, the initial intent, that revenge be taken with the "avenger mak[ing] himself felt" and "with impunity" is definitely achieved. Montresor deceives his victim, Fortunato, into coming to his catacombs where Montresor makes himself "felt" with subtle suggestions and then, finally, his act of revenge in which he tetters Fortunato to a catacomb wall and then builds a false wall that encloses Fortunato into a tomb.  Since no one has discovered this tomb for "over half a century," Montesor has committed his vengeful act with "impunity."  Thus, from beginning to end, Poe's narrator's single purpose of retribution is achieved.

This is a tough question, since there are few positive aspects that arise in "The Cask of Amontillado." I suppose one of them would be "the supreme madness of the carnival season," a time of happiness and frivolity for most of the citizens. However, its decadence also sets the stage for part of Montressor's plans. Montressor's pride of family honor would normally be considered a positive trait in most people, but he takes it much too far. Aside from these minor points, the story tells the tale of two men with few moral values or positive human characteristics.