What is a poem? I'm thinking about this in reference to developments from Modernism on. The writer presents something novel in form with some familiar signs such as appearance on the page, embedded quotations or references, etc. The reader likes or dislikes, but basically accepts. It seems this is a new attitude, less tied to conventional definitions, but is it? Is there still a point to asking, "What is a poem?"
Another panelist should take up this question, but I will start by commending you on appreciating the difficulty of defining 'poetry' given the breadth of sounds and marks that count as poems today. Long gone are the days when 'poetry' could be defined in terms of rhythm, but as we get to the point of having trouble defining boundaries over what is and what is not a poem, we do well to recall that the Greek term (poesis) from which we get in English 'poetry' meant 'to make.' So we may have come full circle. Originally, 'poesis' covered the making of anything; now we may come (sadly or happily) to the same point when almost anything can count as a poem. Even so, there are too alternatives to entertain: define poetry in terms of family resemblence to what is recognized as poetry today. This would mean that a decision whether X (whatever) is a poem is if it resembles the writing of T.S. Eliot, Pound, Edna St. Vincent M, Dylan Thomas (and here follows a long list of poets in the Norton Book of Poetry). This method of definition is sometimes called 'ostensive' and is respectable in different domains: colors have been defined in terms of examples (red, green....) and so have religions (e.g. Y is a religion if it resembles Judaism, Christianity, Islam, Hinduism, Buddhism....). Alternatively, one might try a formal definition. I will leave it to another panelist to do this, except for suggesting that a poem involves language that is about the aesthetic or sonorous character of the words as sounds. To give an example: what makes these lines from the Love Song of Alfred J Prufrock a poem?
"Let us go then, you and I, as the evening is spread out against the sky...."
I think it is not just the referential meeting (someone is asking you to join him or her for a walk at dusk) but this line and those that follow is in some way about the affective (emotively charged) way the language sounds.