Determining what makes a work of literature "good" is tricky because it's very subjective. However, some of the reasons Poe's story "The Fall of the House of Usher" is considered an effective short story include the way Poe is able to set a mood through descriptive writing, how Poe creates suspense, and the techniques Poe uses to ensure that all parts of the story work together toward his purpose.
Poe is a master at setting a mood, and as many of his stories are Gothic, that mood tends to be ominous or spooky. "The Fall of the House of Usher" is a great example of that, and Poe begins building that mood from the start. When the narrator arrives to visit his old acquaintance Roderick Usher, he observes the lake near the home and the Usher's dilapidated estate from a distance. Take the first few lines for instance:
During the whole of a dull, dark, and soundless day in the autumn of the year, when the clouds hung oppressively low in the heavens, I had been passing alone, on horseback, through a singularly dreary tract of country; and at length found myself, as the shades of the evening drew on, within view of the melancholy House of Usher. I know not how it was --but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit...
This is only a small taste of Poe's mood-building at the beginning of the story. Even from this we can tell that the scene is foreboding. The day itself is "Dark" with "clouds [hanging] oppressively low." This environment then presses down upon the narrator himself as he feels "a sense of insufferable gloom" as he views the house. This is only the beginning, so Poe will continue to build onto this creepy feeling as the narrator enters the house, sees the main characters, and starts to hear strange noises during his stay.
The creation of an eerie mood also ties in with a key feature of Poe's writing and of this story: suspense. Beyond the feeling that something is wrong here, just based on the physical appearance of the house and how it affects the narrator, we hear that Madeline Usher has a strange disease that causes her to seem dead when she is not. Roderick also tells the narrator that he expects his sister, and last living relative, to die relatively soon. Once she is "dead," and they move her to a family crypt, Roderick's behavior becomes even more bizarre, and the narrator hears strange sounds, which turn out to be Madeline clawing her way out of the tomb. The suspense crescendos in the final scene, when Madeline emerges, collapses on Roderick, and they both fall dead to the floor. The narrator flees, and the house crumbles around them.
Finally, the story, due to this symbolic ending, pulls together all of the details of the story for a satisfying conclusion. Poe believed that short stories should be read in one sitting and that all elements should be essential and should contribute to the effect the author wishes to achieve. "The Fall of the House of Usher" is one of the most prominent examples of Poe's achievement of his own theory. The double-meaning of the "House of Usher"—both the family home and the family name—leads ultimately to the end of the story, when both meet their demise together: the family and the house die out in one moment. All of the observations the narrator has made about the state of the house and its inhabitants come to fruition in that last scene.
The story works well as a Gothic tale because of the mood and the suspense. The story works as a short story because it is so tightly constructed, and everything contributes to the overall effect. It is also very entertaining to read, and we keep coming back to it over a century after its publication for all of these reasons.