What are the characteristics of Metaphysical Poetry as illustrated in the poetry of John Donne?

Metaphysical poetry is usually characterized by the poet's
attention to a single topic -- usually of a more philosophical nature.  The poets that
are considered metaphysical all display intellectualism and creativity in their poetry. 
The poems are clever and witty as well as interesting in how they present the
speaker's/poet's stance on a subject. 

One of the most
famous poems by John Donne is "The Flea."  In this poem, the speaker is addressing a
young woman and trying to convince her to have sex with him.  His argument centers
around a flea.  At that thought the reader is thinking, "Are you kidding me?  The
speaker thinks a pesky flea is the stuff of romantic seduction???" But that is exactly
what the speaker does (though unsuccessfully.)  The poem becomes a clever and witty
argument that the flea just bit both of the them and therefore their blood is mixed in
the flea and therefore it would be no big deal for their blood to be mixed in the sexual
act (Elizabethans thought that kind of thing happened a lot.)  The young woman in the
poem doesn't accept the argument and she kills the flea, but the reader has to give the
speaker credit for trying this, and the poet for the creative take on how to seduce the
girl.  It is clever, witty, funny, and yet thought provoking at the same time.  That is
what it takes to be a good metaphysical poem. 

Other poems,
such as "A Valediction:  Forbidding Mourning," are clever in a more serious way.  In
this poem, he is telling his wife to quietly accept his upcoming departure on business
by telling her that their love is greater than everyone else's.  He uses metaphors drawn
from science, chemistry, astronomy, geometry, as well as clever language to express his
love.  The most unique metaphor of the poem is his comparison of their relationship to
that of a compass like you would use to draw circles.  His elaboration of the
metaphor creates a conceit.  By taking a common object with no prior associations to
love and crafting this clever metaphor, Donne proves the qualities of the metaphysical
poet here again.