Poetry or Prose - What's the Difference?

"A prose writer gets tired of writing prose, and wants to be a poet. So he begins every line with a capital letter, and keeps on writing prose. "
~ Samuel McChord Crothers

How do we decide whether a piece of writing is poetry or prose? I tend to agree with Crothers (above quote) but there has to be more to it than that.

Dictionary definitions are usually not helpful I find. A short piece of imaginative writing, of a personal nature laid out in lines is often the answer. An art form which represents something of the world, expresses and evokes emotion, is pleasing to read and can be something self-defining perhaps. A piece of writing that shows the cohesion between humans and their lives, even.

I am sure poetry is all these things and more - as an English teacher of mine once said "the best possible words in the best possible order".
But what of prose then?

Ordinary writing as distinguished from verse without metrical structure?
Not all that is classed as poetry has metrical structure, and certainly is not always in verse. Isn't this a subjective view anyway?
Prose can evoke emotion, oneness with the world (or not) and all of the things that "poetry" professes as it's own.

Is there a difference?