Here are some unique aspects:
The sanity (or lack thereof) of the story's narrator: This is what makes the whole story. The narrator begins his tale by trying to convince the reader that he is not mad: "...but why WILL you say that I am mad?" Soon after, he adds: "You fancy me mad. Madmen know nothing. But you should have seen me. You should have seen how wisely I proceeded -- with what caution -- with what foresight, with what dissimulation, I went to work!" The narrator throws comments like these throughout the rest of the story, as he works to remind the reader constantly that he is not mad. What is brilliant about this is that the reader is forced to question the narrator's credibility from the very first few lines and paragraphs of the text. If he is clearly wrong about his judgment of his own sanity, can the reader trust anything he is telling at all?
Suspense. It wouldn't be written by Poe without this. Poe was masterful at developing suspense in his short stories. This one, in particular, is a great example because the pace is literally designed to quicken like a nervous, pounding heartbeat. We see this when he kills the old man: "It grew quicker and quicker, and louder and louder, every instant. The old man's terror must have been extreme! It grew louder, I say, louder every moment!" and again at the end when he confesses: "I felt that I must scream or die! -- and now -- again -- hark! louder! louder! louder! LOUDER!" Of course this is his OWN heartbeat he is hearing, as he grows nervous about being caught.
Tone. The narrator's tone is uniquely erratic because it wavers back and forth between smug and excitable. It is almost comedic at times. For example, he describes the gruesome murder of the old man, and immediately follows it with: "I then smiled gaily, to find the deed so far done." His use of repetition makes him sound like a real person, recounting an actual experience and trying to emphasize the exciting parts.
I've included some links below to help you find more.