I've always found the sort of thing that Michael Rosen says in this video about poetry very unsatisfying.
What is poetry? Well, it's hard to say. Why do people write poetry? To express themselves. Now, to his credit, he doesn't quite leave it at that. But wouldn't it be refreshing if he simply said, "I don't know what poetry is and I'm the wrong guy to make this video. I just can't give you a straight, informative answer." After all, some of the best videojug contributions are precisely those where the presenter confines himmerherself to what heorshe knows well enough to say simple, declarative sentences about.
What would I say in such a video?
Poetry is the art of writing emotions down. A poem is an emotion that has been (more or less) precisely noted. We can then go on to define "emotion" in contrast to "feeling" without at any point having to say, "That's a difficult question."
Why do people write poetry? In order to make an emotional situation more precise, which just means intensifying it. (Compare a conceptual situation, which you make precise by clarifying it.) We can begin with the simple case of a man in love with a woman. His situation is emotionally imprecise because, while she may have smiled at him or returned a glance, it is unclear how she feels. This makes feeling love for her difficult to handle. In writing a poem, the troubadour is hoping to intensify the positions (him-her) at either end of the emotion. She may then, of course, simply reject his advances, but that is more intense than the ambivalence we began with.
All this also applies to Rosen's "big grand things, important things, political things, aspects of nature, the eternal aspects of the sky, the universe or whatever". One may suffer the emotional imprecision of gender (like, say, Sylvia Plath) or those of race (like Amiri Baraka). One may even (like Ezra Pound) try to deal with our monetary emotions, which may be, as they are to today, in something of a disarray. (I made a quick vaguely flarfy attempt here.)
I don't think (as Rosen does) that a good poem needs to offer something "new" or "surprising". After all, a very old poem, read for the hundredth time, is no worse than when it was first written simply because you, the reader, have become familiar with it. A good poem is just emotionally precise; it does not impart feeling, but it intensifies it. It makes you feel better. It makes you better able to feel. I do dare say that this makes the poem good.