Mark Twain's The Adventures of Huckleberry Finn has been called the first modern American novel. Its strengths are found in numerous ways. The narrator, Huck Finn, is virtually unique in American literature: an uneducated, undisciplined poor boy who tells his story in his own unusual language. The novel was one of the first to use native dialects of the South and Mid-West, and Twain's use of colloquial language and humorous satire were also groundsbreaking at the time of its publication. The many underlying themes--including racism, slavery, human rights, prejudice and social consciousness--and symbolism separated it from other adventure novels, including its predecessor, The Adventures of Tom Sawyer. Sadly, the novel has been repressed and dismissed by many critics because of the flagrant use of one word--the "N" word--and it continues to be one of the most banned books in this nation. But most critics still agree with Ernest Hemingway, who probably described it best:
"All modern American literature comes from one book by Mark Twain called Huck Finn... There was nothing before. There has been nothing as good since."
I am sure that there will be a number of different answers to this.
My opinion is that it can be considered great because of the way that it deals with a huge issue in American history -- that of slavery and racism. The book does not shy away from making Jim seem very normally human. He is superstitious (which makes him look bad) but he is also fundamentally good and a very strong person (which makes him look good).
At the same time, the book centers around the theme of individuality. Huck follows his own conscience even when it conflicts with society's values. This is a very American trait -- to be an individual -- at least, that is how we like to see ourselves.