While a poem or song should be able to stand on its own, there are certain assumptions that are made by most critics: The poem is directed at a general audience. This is a stand-alone work. Etc.

When you put out your work for critique, either by e-mail or on a poetry board or handing a copy to a friend, give a little background information. Who is the intended audience? Is this stand-alone, part of a series, or both? Is it formal of free? If it is formal, what form? Are there any other requirements or information the critic needs? By making this sort of information explicit for the critics, you can end run many questions and comments that might not be appropriate.