I read a good definition of poetry this semester.

Samuel T. Colderidge's definition, found in his Biographia Literaria, is, in my opinion, pretty spot on. His view is that poetry is the most pleasurable form of written expression, and this is because it is an organic whole. That is, every part plays into every part, and the unified idea is thereby communicated the most effectively. The pleasure comes from looking into the details and finding, again and again, more of the whole: the idea (hopefully a truth) that the poem communicates.

The problem for the Romantics in defining poetry was the use of meter. Coleridge's definition resolves this: meter is a way of organizing words such that every one has a proper place. This allows the words, in a way that a novel can never achieve, to each have an ideal place, a place where they can communicate the idea most effectively through rhythm and rhyme, etc. Meter also allows one to create a feel that prose cannot, for example, the Rhyme of the Ancient Mariner is written as a mariner's ballad. This kind of poetry is intrinsic to a certain kind of England, and creates the character of the Mariner. It creates a time and a place through which the character can be understood while minimizing narrative time.

That's why poetry can be so condensed, and why dense poetry is so pleasurable, it's food for thought for days and days.