I've done three critical studies of this book in my life, HS, undergrad, and grad. As a whole, the novel is great in chunks. As a complete work, it is lacking some modern workings of the novel that would develop later in the 19th century. Some of my favorite parts are the chapters with the Dauphin and the family argument but I always have issues with this novel from a critical viewpoint. 
 1. Why does Jim have to disappear and reappear? It hurts the novel's 
 Attack on the institutions of slavery and racism. 
 2. Why not push the youthful point of innocence further in the plot? Huck grows up to early in the structure of the novel. 
 3. Why have him go home? If Huck truly were the great protagonist America wishes to be why return to what we were, because of fear. That makes for a weak purpose.