What constitutes poetry?

Here's a philosophical question for you. What makes a poem a poem?

We all remember high school English class where we learned about iambic pentameter and rhyming schemes and even wrote our own poems to be graded. But are rhythm and rhyme all there are to a poem? I don't think so.

Remember free verse? Yeah, that "poetry" where you, essentially, took a prose paragraph and broke it into smaller lines to make it look like a poem. It didn't have to rhyme. It didn't have to have a regular rhythm. It was free verse. You could do anything you wanted.

And what about forms like haiku? It doesn't rhyme, and its rhythm is simply a number of syllables. The accents could be anywhere. It could be just a series of words or a sentence broken into lines. I've seen haiku like that from the New York Times. It's interesting to see the "found haiku" they share on Twitter.

But what is the heart of a poem? Is it the rhythm? Is it the rhyming scheme? Or is it, perhaps, the subject matter? The poems we studied in high school supposedly had deep meanings hidden in innocuous symbolism. I'm still not sure how Emily Dickinson, for example, filled her poems with such deep meaning. After all, "Because I could not stop for Death, he kindly stopped for me" is pretty straightforward.

Maybe a poet's gift is in the choice of words, the combinations placed on a page and sent out for all the world to see and for every English professor to dissect until the poet's original work has vanished.

I don't have an answer to my opening question. I've never had any aspirations to being a poet. I got frustrated when teachers "explained" the deep hidden meanings in a piece of poetry. I always wanted to ask "What if the poet didn't have an agenda when he wrote this poem? What if it just means what the words on the page say?" I'm sure I would have been sent to poetry purgatory and been forced to read Vogon poetry for days on end. Actually, that would be poetry hell.

I've often wondered who was the first person to "discover" a hidden meaning in a poem. What made them think the poem meant something else? How did they go about mining for symbols and their meanings? And do poets think about hiding messages in their works? Or do they think these discovered meanings are funny? I can just hear Shakespeare laughing with Homer - "Can you believe what they think that sonnet means?" - to which Homer replies - "I know, right? How about the symbolism of the Trojan horse? It's just a horse the Greeks hid in to defeat Troy. And I didn't even mention it!"