Samuel Johnson’s edition of William Shakespeare’s plays took him decades to bring about. Around 1745, Johnson got the idea to put together a multivolume edition of Shakespeare’s works. In 1765, that work finally came to fruition. The long delay made him vulnerable to mockery. The English satirical poet Charles Churchill quipped, “He for Subscribers baits his hook / And takes their cash—But where’s the Book?”

However, as the question notes, the painstaking edition ultimately earned Johnson extraordinary praise. According to Samuel Johnson’s biographer, James Boswell, England was ridiculed by the outside world for its “blind, indiscriminate admiration of Shakespeare.” Johnson’s preface, in which, according to Boswell, “the excellencies and defects of that immortal bard are displayed with a masterly hand,” provided a pivotal counter to the gratuitous acclaim.

In a sense, Johnson’s criticism of Shakespeare takes on landmark status because of its context. Johnson’s balanced survey of Shakespeare went against the adulation of the time and opened the playwright up to fresh interpretations. Considering the large role of Shakespeare in English culture, Johnson’s judicious reading caused quite a bit of commotion.

As for the actual ideas presented in the preface itself, they could be called landmark because of how they made Shakespeare more accessible to everyday people.

For instance, Johnson presents the characters in Shakespeare’s plays as relatable. Johnson claims that there are no "heroes” in the works. What one finds is “men,” or flawed, everyday people who “act and think” as the reader might.

As alluded to earlier, Johnson also brings Shakespeare down a notch. He castigates Shakespeare for lacking virtue. Johnson speculates that if Shakespeare would have tried harder sometimes, some of his plays might not be so mediocre. Again, such criticism was not exactly common at the time, which is why Johnson’s preface is seen as a landmark in Shakespearian criticism.

Johnson's Preface to Shakespeare is a landmark in that it is not a hagiography but a balanced attempt to assess the work of a great playwright. A cult of veneration had already grown up around Shakespeare before the publication of Johnson's edition of the plays in late 1765 (it did take Johnson about twenty years to get this work in print). Johnson adds his own imprimatur to the Shakespeare veneration by throwing his reputation and weight behind the plays, writing that Shakespeare

may now begin to assume the dignity of an ancient, and claim the privilege of established fame.

However, and this is key to what makes the preface a landmark, Johnson does not just praise but makes a careful critical assessment of Shakespeare's work. He focuses on the work itself rather than the author's life or sources. He defends the plays against criticism that they violated Aristotle's unities of action, time, and place, legitimizing the idea that a genius can break the rules and still soar. Johnson asserts that Shakespeare exhibits a higher form of realism based on the remarkably true to life characters he creates. That ability to make his characters come alive transcends the need for adhering to rigid rules.

Johnson's positive focus on Shakespeare's characterization as well as his critique of Shakespeare's vulgar or low humor would have a deep influence on nineteenth-century writers' and critics' understanding of the playwright.

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.