The term "metaphysical" as originally coined by Dr. Johnson was intended to be used as a pejorative. By the time Johnson came to write his Lives Of The Most Eminent English Poets,the works of the metaphysicals had become somewhat passée, and it is in the course of his chapter on Abraham Cowley that Johnson ably summarizes the general disdain for the metaphysical school which existed in the world of English letters at that particular time.
"Metaphysical," for Johnson, is primarily conceived in stylistic terms; he deems the metaphysical style to be too elaborate and over-refined, using intricately contrived verse to convey abstract reflections on a variety of subjects, characterized as philosophical in the broadest sense of the word.
At the heart of Johnson's criticism is his marked disdain for the conceit. This is a key element of metaphysical poetry, one that has, down the centuries, drawn both considerable praise and censure in fairly equal measure. Essentially, a conceit is a variety of analogy, one intended to convey a particularly novel and striking idea through the use of figurative language. To that end a conceit differs from more traditional analogies in that the analogy used and the thing with which it is compared do not appear to have an obvious connection. This is what Johnson is referring to when he complains that "The most heterogeneous ideas are yoked by violence together."
Arguably the most famous conceit (or infamous, depending on one's evaluation of metaphysical poetry) is provided by John Donne in his much-anthologized poem "The Flea." Here, Donne seeks to make a highly suggestive comparison between the bite of a flea and the act of copulation. In the seventeenth century, sex was commonly conceived as a mingling of the blood, so Donne feels free to compare the sense of oneness achieved through lovemaking with the unity achieved between the flea and his hapless victim. If the flea can mingle his blood with the suitor's lady, then why can't the suitor? For just as the lady will not suffer death through her unfortunate encounter with the flea, nor will she lose her honor by submitting to her lover's advances.