I recently judged a contest for adult writers writing for children. Unfortunately, a significant number of those entries did not use standard manuscript format. It's not difficult to find out how a manuscript should be formatted--it's in market books, on the Internet, in books about writing--but some writers are ignorant of the basics.
I was paid to read all the submissions even if they were in too small a font and/or not double spaced, or if paragraphs weren't indented. I can tell you, though, if I'd been an editor, those would have hit the recycle bin without any hesitation. I'd probably be more patient with narrow margins, although as an instructor, I do get frustrated when the margins are too narrow for me to write in. Does that frustrate editors, too? I'm guessing yes.
Surprisingly many articles--good ones, too--did not provide word count at the top of the manuscript. Again, a standard format practice. Yes, I could guess how many words a manuscript is by the number of pages . . . at least if the font isn't unusual or too small . . . but if I was a magazine editor, the word count might give me an indication right away whether the writer looked at my guidelines or not.
Some writers were trying too hard to impress . . . with the type of paper they used. Don't use résumé paper, watermark paper, paper with borders for manuscript submissions. Pay attention to what's more important--the content.
Over and over again I've heard editors and agents say that too many typos and misspellings says to them that the writer didn't care enough about what they've written. If the first page or two has errors why would it get better later? I didn't have the luxury of not reading on, but I can say the finalists had much fewer errors, if any.
So, if you care about your writing (why send it out if you don't!), make sure you follow the simple rules of standard manuscript format. Triple check for typos, spelling, and grammar errors. Give your material a chance to be read just by following the basics.
P.S. And for those not familiar with standard manuscript format, I'm providing it here. The text of my sample first page includes more tips. manuscript format.pdf
P.P.S. Oh, and please don't staple your manuscripts!