The emphasis on the use of the vernacular throughout the story reinforces the sense that it deals with people who are generally removed from more mainstream, polite society. The language of the of run-down, sordid little towns along the river, the devious king and duke, and of the African Americans all serve as a counterbalance to the more genteel, standard English that was used in most American writing previously. It gives a greater air of authenticity and vividness to the portrayal of those segments of society. This is the main effect it has on the reader, although those unfamiliar with such dialects may also find this aspect of the novel somewhat off-putting.
This novel really marked something of a new departure in not only showcasing the use of American vernacular but also having, as a narrator, someone who didn't talk correct, formal English. Huck's dialect is not so marked as some of the characters he encounters, but it is still far enough removed from standard English to be quite notable, and he uses slang liberally throughout. In addition, he is still only a teenager; his voice is entirely informal, colloquial, and not in the least concerned with conforming to a standard usage of English. Indeed, being forced to use standard, polite English all the time has been one of his chief frustrations when living with the Widow Douglas, whose valiant attempts to civilize him don't achieve very much.
It has been observed that the use of the vernacular in this novel helped to develop an unmistakably American idiom in literature, and it is regarded as one of the most important American novels just in light of that fact.