This novel is a masterpiece. The prose is clean. The characters are both real and symbolic.
The greatest quality of Heart of Darkness is its depth. You can read this book ten times and still feel that there is more to understand, more to dig into.
For a small novel, there is quite a bit of intellectual material. That is admirable.
There is also a sense that this little book really attempts to say something signficant about the existence of man (and/or about the existential side of the human mind). That, also, is admirable.
Heart of Darkness is admirable book on many, many levels. The expressive language and complex style are virtuosic, especially considering the Joseph Conrad is writing in his third language. The content and the analysis of the colonization of Africa is as insightful and rich as that written by any historian or economist. As a piece of literature, if we take E.A. Poe's suggestion that every element of a work should be directed at producing a "singular effect" in the reader, I think we see a prime example in this novella.
Aside from the beauty of the book, and the depths of the themes and ideas, Conrad himself makes this book remarkable. The man was born in Poland, and did not learn English until he was in his twenties. His command of the English language is stunning regardless, but given this fact, it is even more impressive.
A great deal is admirable about "Heart of Darkness." Two interwoven points come immediately to mind. First, the writing style. The line by line prose and the many symbols are incredibly rich and evocative. For example, look at this line from early in the work: " Flames glided in the river, small green flames, red flames, white flames, pursuing, overtaking, joining, crossing each other -- then separating slowly or hastily." Man! That's vivid. A second point is the insight into the human psyche: how we each create our own hearts of darkness.