Emerson’s remark helps suggest the all-important difference: “It is not meters, but a meter-making argument that makes a poem.” In this kind of poem the poet establishes regularity only to depart from it expressively. When he does compose a metrically regular line it is not because the metrical scheme tells him to, but because something in the matter he is embodying impels him toward a momentary regularity.   […] 

The poet working in free verse has already chosen to eschew one of the the most basic expressive techniques in poetry.

Paul Fussell: Poetic Meter and Poetic Form
(from Chapter 3, Metrical Variations)