"I'm Nobody!" is one of Dickinson’s most popular poems, Harold Bloom writes, because it addresses “a universal feeling of being on the outside”. It is a poem about "us against them", it challenges authority (the somebodies), and "seduces the reader into complicity with its writer."
Great poet! Seen some beautiful poems written by her. Now, I know someone who won two poetry awards and let me tell you a secret about what makes a poem good. It is two things really: the feeling it awakens and the imagery it conjures up. If the poet is talking about walking through a meadow at sunrise and you can "see it" in your mind, he or she has done her job. If the poet is talking about the pain of a broken relationship and you suddenly remember someone you dated, loved and lost 5 or 10 years ago, that is one really good poet! It is exactly the same as with prose.