Really, the guidelines are there for a reason.
I don't mean vague things like "themes" or genre elements or stuff like that.
I mean basic things.
For example, I've written twice about cover letters (here, and more recently here). The latter post is explicitly linked to from both the guidelines page for recompose and in the actual submissions manager itself. For clarity's sake, here's a screencap of Leslie's example of what she wants to see as a cover letter:
And yet I had reason to make this meme:
Not joking, folks. Seven pages, single spaced.
|blurred and anonymized, but otherwise untouched|
Contrast those two cover letters, if you will.
On top of everything else, it was a reprint, after we stopped taking reprints. And sent by e-mail, after the submissions manager was put in place.
Did I notice it? Oh yes... but not really in a good way.
Remember, your cover letter serves to introduce your story (or poem), not take the place of it. It gives your contact information, what the work is, and if relevant, what credentials you have for writing the work. Beyond that is the work of the story or poem, not the cover letter.
(And following the rest of the guidelines is probably a good idea, too.)