In The Lives of the Poets, Samuel Johnson clearly argues that the metaphysical poets exhibit wit by joining incompatible ideas in order to create startling images, Johnson also concludes that the reader "commonly thinks this improvement dearly bought, and though he sometimes admires, is seldom pleased."
The metaphysical poets comprise a diverse group of poets--from John Donne to Robert Herrick and Andrew Marvell--and the hallmark of their poetry is the use of unusual imagery and shocking poetic conceits to startle the reader into understanding.
For example, in a great example of the carpe diem ("seize the day") theme, Andrew Marvell, in his very funny poem "To His Coy Mistress," argues that his lover must stop playing games and get down to the love business:
They beauty shall no more be found,/Nor in thy marble vault shall sound/My echoing song; then worms shall try/Thy long preserved virginity. (ll. 25-28)
Here, Marvell is yoking very different ("heterogeneous") ideas--the passing of time and development of love with gruesome physical decay--to create a violent image of virginity being attacked by worms that, in Johnson's view, may be witty and shocking, but ultimately too grotesque for the reader.
The horrifying prospect of a young woman's virginity being attacked in the grave creates certainly a surprising image that might arrest the reader's attention for a moment, but Johnson argues in his treatise on the metaphysical poets that such wittiness ultimately violates the reader's sense of the orderliness of the world. In other words, shock for shock's sake does not further one's understanding of poetry.