It's hard to describe exactly what a great poem does. Here are some thoughts:
"If I feel physically as if the top of my head were taken off, I know that is poetry."
"Energy within the poet goes into the poem, but then must go from the poem to a reader or listener. There has to be this transfer of energy."
"Imaginative work is not dropped like a pebble upon the ground; it is like a spider web attached ever so lightly, but attached to all four corners of the earth."
"Like a piece of ice on a hot stove, the poem must ride on its own melting."
"Truth is not an unveiling which destroys the secret, but a revelation that does it justice."