The answer above is a great one and really speaks to the importance of the content of this novella and its expression of one of the central elements of the human experience: our understanding of good and evil.
To this, I would also add that Heart of Darknessis an expertly crafted example of the artistry of literature. The themes, imagery, and symbolism are all tightly wound and all contribute to the central theme and idea presented in this work. This novella is exemplar of literary 'high art.'
Also, the novella stimulates an emotional response in almost everyone who reads it. Conrad himself often criticized his lack of a central message or moral to this story, but believed the reading experience of this novel could have a profound and sometimes transformational impact upon the reader.
For all of these reasons, Heart of Darkness appeals to readers and remains a key title in our literary canon.
You can and just might pick up a great many answers to this question. Certainly, all of the standards you mention by which you can judge the work could help to create its long lasting impact in making it a rather inseparable part of the Western Literature Canon. In my mind, the primary source of its appeal lies in how the work speaks to what it means to be human. The idea of a civil war raging in the individual between what is socially conditioned and a nature that might belie such a contrived appearance is a rather powerful element. Both components are part of the individual, and a setting where challenge in assessment is evident. In this light, the book represents what Colonialism actually way. Centuries had been spent extolling the virtues of the age of exploration and how these brave individuals set out against all odds in searching for glory and expanding the concept of the world as it is known. Beneath such a brilliant facade lies the treatment of indigenous people and "the horror" of what was done under the guise of "civilization." I have always felt that reading Conrad's work along side Chinua Achebe's "Things Fall Apart" can probably go very far in detailing what the term "colonialism" means and how the relationship between "The West" and the rest of the world has progressed over time. I can only feel that the reason why Conrad's work speaks for so long and in such a powerful manner is because of how it opens the dialogue and the discourse of what it means to be human and to be a product of history. Its self reflective nature of what it means to be "the West" is also resonant.