In "The Fall of the House of Usher," what does the author find striking about Madeline's dead body?

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.