for the reader to be able to read the poem over and over again and still be able to find some new message in it, to have meaning to the poet and for someone to be able to connect to a deep level of emotion with it, and if it's describing a feeling, than the reader needs to feel as if they are truly in the moment, or if you try to describe summer time, than you should be able to almost taste the summer wind and imagine the way the sunshine feels on your skin. not all poems need to have exactly these things because they are like fingerprints, each one is different and describes a different person. hope this helps 