Dr. Johnson's aesthetic or idea of what made poetry good was formed by the standards of the eighteenth century, which valued poetry that imitated other poetry, especially the classical authors of Greek and Rome, had a smooth, regular rhyme scheme that fell easily on the ear (this was the age of the heroic couplet, two rhyming lines written in iambic pentameter), and poetry that touched the emotions. 

To Johnson, the metaphysical poet violated all these rules. Their poetry used bizarre or unusual metaphors, strange, jagged rhyme schemes that were not regular, and the poems, failed, in his opinion, to touch the emotions. He thought of them as making clever efforts meant to impress rather than to be easily understandable. These poets strutted their stuff but were not worth the effort it took to decipher them. As Johnson put it:

The most heterogeneous ideas are yoked by violence together; nature and art are ransacked for illustrations, comparisons, and allusions; their learning instructs and their subtlety surprises; but the reader commonly thinks his improvement dearly bought, and, though he sometimes admires, is seldom pleased.

In his essay on metaphysical poets, Johnson does not single out Donne, but we can easily imagine that Donne, the most famous of the metaphysicals, would be a prime example of a poet who used the kind of tortured metaphors and uneasy rhyme schemes that Johnson so disliked. For example, Donne compares lovers to saints, a comparison considered odd at the time, and likens lovers who are separated to the legs of a compass. His rhymes are often uneven, imitating the cadences of spoken language. 

Today, our aesthetics have again changed, and we admire the creative originality of the metaphysical poets.