Samuel Johnson is the greatest prose stylist in the English language, and reading his criticism on Shakespeare is a rewarding, though difficult, education. His vocabulary is as advanced as the language permits, which is fitting given that this is the man who single-handedly wrote the English dictionary. Some sentences require rereading and a good dictionary to decipher. The effort is worth it. The prose, once unlocked, has a weight and balance and moral seriousness that rewards the patience it demands.
The critical intelligence behind that prose is formidable — rigorous, honest, deeply humane, and filled with common sense.
Johnson stands for common sense, correctness, established systems, and accepted rules of decency and propriety. These are genuine virtues, and they make him an extraordinarily reliable guide to much of literature. But Shakespeare is not a writer who submits to those values. he flouted them, bent language to shapes it had not previously taken, violated accepted morality when his dramatic purposes required it, and used words in ways that struck language attuned readers as simply incorrect. Johnson is frequently bothered by this. He notes Shakespeare's misuse of words, his irregularities of construction, his willingness to sacrifice grammatical propriety to dramatic effect, his moral unorthodoxy.
What Johnson cannot quite see, or perhaps forgive, is that these are not failures. They are the mechanism of the genius. Both fools and geniuses break out of the box of convention and acceptability; what distinguishes them is what they find outside it. Johnson keeps misreading Shakespeare's unorthodoxy as foolishness when it is rather the evidence of a mind that simply operated at a register beyond the reach of rules. You do not read Shakespeare for correct prose. You read him for what correct prose cannot do.
The morality question is related. Johnson believes, with great sincerity and consistency, that literature ought to be morally instructive. A view similar to Sir Philip's Sidney "A Defence of Poetry", Johnson thought literature should teach virtue, that justice should be delivered, that vice should be punished and virtue rewarded. Shakespeare notoriously does neither with any reliability. The moral architecture of the plays is too complex, too honest, too tragic to deliver the clean lessons Johnson wants from them. Johnson's critical criticism is simply not a position adequate to the writer he is judging.
Johnson also believes that Shakespeare excels at comedy but falters at tragedy. I wholeheartedly believe the opposite. Nonetheless to spar intellectually with an intellectual superior of a critic is a great education. Read him slowly, dictionary at hand (perhaps even Johnson's), and argue back, with due reverence to this elder man of letters.