I am guessing that you want to know what the narrator (and not the author) finds striking about Madeline's dead body.  (This is because Poe himself is not a character in the story.)  That being the case, the narrator finds two things striking:  the resemblance between Madeline and Usher as well as her rosy hue.  First, let's deal with the uncanny resemblance:

A striking similitude between the brother and sister now first arrested my attention:  and Usher, divining perhaps, my thoughts, murmured out some few words from which I learned tha the deceased and himself had been twins, and that sympathies of a scarcely intelligible nature had always existed between them.

Usher and Madeline, then, were fraternal twins who look very much alike.  Could this be a similarity that existed from birth or since the advent of the "malady" they both share?  Your guess is a good as mine.  In addition, there is a definite mention here of a special camaraderie the two shared:  one of a "scarcely intelligible nature" so as not to be ascertained by anyone else.  This kind of connection is often the case between twins, but one wonders if there is a greater and even supernatural connection here.

Secondly, the narrator is surprised further by how Madeline looks:

The disease which had thus entombed the lady in the maturity of youth, had left, as usual in all maladies of a strictly cataleptical character, the mockery of a faint blush upon the bosom and the face, and that suspiciously lingering smile upon the lip which is so terrible in death.

Not only does Madeline look young but also rosy-cheeked in death!  This is our first sign that perhaps Madeline is not dead at all.  In fact, the mention of the "faint blush upon the bosom and the face" is reminiscent of Juliet before she wakes in her tomb (after consuming a poison to produce a sound sleep that seemed like death).  Sure enough, Usher later admits that, "We have put her living in the tomb!"

The fact that the narrator notices both of these things plants a seed in the reader's mind both in making us wonder whether Usher and his sister are one in the same person and in making us wonder if Madeline's living entombment was purposeful.

What does the narrator find striking or unusual about Madeline's dead body?

As the narrator and Usher lay Madeline's body in the family vault, the narrator notes several queer details about the corpse before him.  First, he notes how strikingly Madeline resembles her brother.  Just as he notices this, it seems Usher reads his thoughts and murmurs that he and his sister were twins.  Probably the more notable details, however, come in these lines:

Our glances, however, rested not long upon the dead—for we could not regard her unawed. The disease which had thus entombed the lady in the maturity of youth, had left, as usual in all maladies of a strictly cataleptical character, the mockery of a faint blush upon the bosom and the face, and that suspiciously lingering smile upon the lip which is so terrible in death.

Here, the narrator notes that as a result of her disease (which, though undiagnosed by doctors is one that causes her catalyptical seizures, or physical fits which leave her entire body rigid, as if she is dead) she has died with a faint blush upon her bosom and face, and seems to be smiling.  A normal corpse would of course typically be devoid of color, as the blood would no longer be pulsing throughout the body.  Also, it is particularly strange and eerie to the narrator that Madeline seems have died with a smile on her face.

All of these details are particularly important when you realize that they are foreshadowing a most bizarre and ultimately creepy end to the story.  Madeline, at this moment, is not actually dead.  She is simply in the throws of such a seizure, which is apparently severe enough to have slowed her heartbeat and breathing to an undetectable state.  Note that at the climax of this ghost story we see:

...there did stand the lofty and enshrouded figure of the lady Madeline of Usher. There was blood upon her white robes, and the evidence of some bitter struggle upon every portion of her emaciated frame.