Although The Outsiders has been a favorite with teens ever since its publication in 1967, adult critics have been more cautious in their assessments. Initial reviews debated the supposed "realism" of this startling new work, as well as the skill of its young author. Thomas Fleming, for instance, questioned Hinton's portrayal of the Soc-Greaser conflict. He noted that in his hometown it was the poor kids who beat up the rich ones, not the other way around. Nevertheless, he added in his New York Times Book Review assessment that "Hinton's fire-engine pace does not give the reader much time to manufacture doubts." Nat Hentoff similarly observed in Atlantic Monthly that the plot of the book was "factitious," or forced and artificial. He praised the author, however, for addressing issues of class that were absent in previous books for teens: "Any teenager, no matter what some of his textbooks say, knows that this is decidedly not a classless society." School Library Journal contributor Lillian Gerhardt similarly hailed Hinton's portrayal of class rivalry: "It is rare-to-unique among juvenile books ... to find a novel confronting class hostilities which have intensified since the Depression." In another early review, William Jay Jacobs favorably compared The Outsiders with a popular classic from the 1950s, J. D. Salinger's Catcher in the Rye. Other critics have observed similarities between Hinton's Ponyboy and Salinger's Holden Caufield. "But as much as the sensitive, thoughtful Ponyboy resembles Holden, his [environment] is irrevocably different," Jacobs noted in Teachers College Record. "All around him are hostility and fear, along with distrust for the 'system.'" The critic did fault some of the dialogue as "false" and the themes as a bit too "profound" for "hoods." Nevertheless, he noted that the novel had a more mature tone than most first novels, and had "relevance for today's [society]."

By 1970, The Outsiders had already been identified as a powerful influence on young-adult literature. Many critics questioned whether it and other examples of the "New Realism" were a positive influence on teens. Attempts to ban the book were made in various places. As a result, many reviews of the time were particularly negative. For instance, a Times Literary Supplement critic worried that young readers "will waive literary discriminations about a book of this kind and adopt Ponyboy as a kind of folk hero for both his exploits and his dialogue." Other critics faulted the slang dialogue and sometimes moralizing tone. In his Children's Book News review, Aidan Chambers noted that the book was flawed because it was written with self-indulgence "and could profitably have been cut." Nevertheless, reviewers could not deny the appeal the book had for teen readers. As Chambers added, the first-person narrative had "interesting qualities," such as compassion and lots of action.

Critics have also recognized, however, that the strength of The Outsiders lies in its characters. In her 1969 work Children's Reading in the Home, May Hill Arbuthnot praises the book's "incisive portraits of individual boys growing up in a hostile environment ... The characters are unforgettable." Alethea K. Helbig and Agnes Regan Perkins made a similar observation in their 1986 work Dictionary of American Children's Fiction. They remarked that while some of the incidents in the plot seem unbelievable, "they hold up well during reading, probably because the author makes Pony's concerns and the warm relationship between the brothers seem very real." Cynthia Rose likewise stated in Monthly Film Bulletin that Hinton's "characterisation of the emotional claustrophobia and relentlessly limited prospects of the poor white world—where sacrifice so often defines love—is her most impressive literary achievement."

Hinton's novel has maintained its popularity for over thirty years, leading later critics to analyze its appeal. In a 1986 Nation article, Michael Malone suggested that it was because The Outsiders conforms to the popular myth of "the tragic beauty of violent youth." He observed that rather than being realistic, Ponyboy's language and story belong to a mythic or ideal world where teens anguish over their problems without adults to hinder or help them. Nevertheless, Malone added that Hinton's ability "to evoke for her audience how teenagers feel about those clashes [of ideals] is indisputable." On the other hand, critic Michele Landsberg called Ponyboy's many poetic descriptions, particularly those of the greasers' appearance, "simply absurd." She explained in her Reading for the Love of It that Hinton's book "flatters the egos of young male readers with its barely-subliminal sexual praise, and lets them escape into the fantasized glory of attention and approval from an older teenage tough."

Other critics have found true literary merit in the novel, however, merit that explains its long-lasting popularity. As Jay Daly observed in his Presenting S. E. Hinton: "It has nothing to do with the age of the author, and little to do with the so-called 'realism' of the setting. It does, however, have very much to do with the characters she creates, their humanity, and it has everything to do with her honesty." "One of Susan Hinton's significant achievements in The Outsiders is to hold up for scrutiny young people from economically, culturally, and socially deprived circumstances," John S. Simmons claimed in Censored Books: Critical Viewpoints. "In Ponyboy Curtis, his brothers Sodapop and Darry, and his 'Greaser' companions, Hinton has introduced readers, most of whom have probably been from white, middle class origins, to the desires, the priorities, the frustrations, the preoccupations, and above all, the anger of those young people who may live in the seedier parts of town but who have established a code of behavior which reflects (to the dismay of some) their sense of dignity and self-worth ... Most important, they believe in, trust, and support each other, all sentiments which can be universally admired despite the circumstances in which they are displayed."

Hinton herself has always known the key to her success. "Teenagers should not be written down to," she wrote in the New York Times Book Review upon the publication of The Outsiders. As a result, Hinton is an amazingly popular writer among teens and, especially, reluctant readers. Librarians and teachers use her books frequently for reading assignments. Hinton sums up the attraction to her action-packed gang thrillers, saying: "Anyone can tell when [a teen's] intelligence is being underestimated. Those who are not ready for adult novels can easily have their love of reading killed by the inane junk lining the teenage shelf in the library." So she has gained the devotion of teen readers by following her own advice: "Earn respect by giving it."