Johnson's "Preface to Shakespeare" is a landmark in Shakespearean criticism for a number of reasons. First, it is a balanced critique of Shakespeare, giving him credit for his powerful and lyrical use of the English language, praising his examination of fundamental and timeless human emotions, and admiring his iconoclastic departure from convention even as it took issue with his use of plot structure, anachronisms, and vulgar double entendres and puns.
Second, Johnson's focus on character development as a major aspect of Shakespeare's genius would serve (and indeed still serves) as a focus of criticism for successive writers. Samuel Taylor Coleridge and other famous critics would follow many of the same lines as Johnson.
Third, Johnson took Shakespeare seriously as a writer, defending him from the condescension of many continental critics who saw his departure from classical forms (such as Aristotle's unities) as an unpardonable sin. This would serve as a plaform for other defenders of Shakespeare such as Victor Hugo.
Overall, Johnson provided a paradigm for future critics of Shakespeare even as he asserted the importance of Shakespeare among western writers. His work was important in establishing the Bard's now virtually undisputed place in the western canon. As Johnson observed:
The Poet, of whose works I have undertaken the revision, may now begin to assume the dignity of an ancient, and claim the privilege of established fame and prescriptive veneration. He has long outlived his century, the term commonly fixed as the test of literary merit.
Johnson's work helped ensure that he would continue to endure in importance.