I've been reading slush--more slowly than I'd like--trying to get caught up. Recently I read a piece in which the voice bogged the prose. It was an interesting voice and intriguing story line. The dialect and even the subject matter put me in place immediately. But, dialect and word choice is like setting--select details can go a long way to put the reader in place without interfering with the story.
Weighing a story with too much of anything can manifest itself in all sorts of ways, of course: the fantasy that describes rooms and clothes in pages of great detail or historical accounts of why a military science fiction character can't pull the trigger (right in the middle of a battle--yup, I've seen it). Sometimes it's just the author not recognizing the difference between what they needed to discover about the story and characters in early drafts verses what a reader needs to know in a final draft. (I call that authorial discovery, and it's often the stuff that must be cut later.) Knowing "when to say when" comes with experience, I think, and all of us throw the dice on which details to include or take out. However, employing a distinctive dialect in narrative is perhaps one of the biggest gambles a writer can take.
This is not to say that the next editor won't love that voice, but this editor didn't, for a variety of reasons. As an editor, I can look at back story and description and determine easily what serves the story and what can be cut. But voice and dialect in narrative are different--they infuse every sentence to the degree that paring it down becomes a major undertaking. And, remember, all I see is your story. From your prose, it's my job to determine how easy you'll be to work with and how readily you'll accept my editing. Every time I hit send on my editorial notes I wonder vaguely if the author will be offended. We've been fortunate to work with professionals in that regard, but it is one of the reasons we don't go to the trouble to ask for rewrites from our slush.
But, when a voice overpowers the prose, I have to assume the writer is too in-love with it to see it taken down a notch. Knowing what must be cut is a sign of moving forward in writing. Sometimes I think, like in marriage, successful storytelling is more about what you DON'T say.