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.