Johnson is not a biased critic of Shakespeare in his Preface. He carries with him, as any critic does, certain standards by which he judges literature, but he hasn't left out or distorted evidence to make Shakespeare fit a predetermined judgement, as a biased person would do. In fact, Johnson explicitly states that he is trying to get beyond the contemporary bias in favor of ancient literature over modern in judging Shakespeare's worth. As Shakespeare is considered a modern writer (meaning he is not a writer from ancient Greece or Rome) and therefore likely to be too harshly judged, Johnson is at pains to judge him fairly.

Throughout the preface, Johnson does exactly what an unbiased critic is supposed to do: he weighs Shakespeare's work on its own merits, studying it carefully and in detail. Johnson finds it on the whole praiseworthy. He commends the realism of Shakespeare's characters, who represent a mixture of good traits and flaws, and he doesn't condemn the plays for mixing comedy and tragedy together, because that reflects life.

While Johnson concedes that Shakespeare violates the unities of time and place, he nevertheless carefully examines what Shakespeare is doing. Johnson defends him, for "he has well enough preserved the unity of action." In other words, Johnson tries not to fall into the bias of condemning Shakespeare out of hand for violating an arbitrary rule about unity. Shakespeare's plots make sense and hold together logically, with one scene emerging out of another, which is what the unities are trying to ensure. Therefore, it does much matter that he ignores the rules, Johnson argues.

Johnson, being unbiased, criticizes as well as praises Shakespeare. He says he is often too wordy. He dislikes what he thinks is Shakespeare's excessive use of puns, which he calls "quibbles," stating that they have "some malignant power over his mind."

All in all, while coming down on the side of Shakespeare as a very fine playwright, Johnson judiciously considers his work from many angles to try to assess him fairly.

Johnson is biased in the sense that he generally favours the neo-classical standards of criticism of his day, that is to say, standards deriving from the ideas of ancient classical writers. This leads him to express disapproval of certain features of Shakespeare's plays - features which are nowadays regarded as being some of Shakespeare's greatest strengths. Johnson recognises the power of the plays in presenting an intense and truthful view of human nature and is true to his classical values in praising Shakespeare for dealing in universal truths, but also castigates him for a lack of morality in his plays. For instance, in the tragedies, virtue is not seen to be rewarded. Johnson's neo-classical ideals led him to believe that the main purpose of art is to morally instruct whereas he sees Shakespeare as being primarily an entertainer. Johnson also criticises Shakespeare for not observing the dramatic unities of time, place and action as proposed by the influential Ancient Greek critic Aristotle and frowns upon Shakespeare's excessive punning and wordplay. Such things do not trouble the majority of critics today.

However, overall, Johnson does recognize and praise the great power of Shakespeare's work and his Preface to Shakespeare did much to bring Shakespeare's plays to the forefront of critical attention - where, of course, they remain today.