One thing important thing that has come through to me from reading recent manuscrpts is how important it is for the reader to feel that they are in safe hands, that the writer is confidently taking them somewhere. I think that is part of the appeal of the journey as a narrative device. And it's the problem with too much back story at the start - the reader doesn't feel that there being taken anywhere. And too much back story at the start is one of the commonest flaws in the unsolicited manuscripts.
Sometimes I feel the writer can be indulging themselves at the expense of the reader by putting in a section, a piece of writing, or a chapter that's something they want to put in rather than something that the story needs. They may particularly like this piece of writing and want it to be read and admired but it floats on top rather than being part of the flow of the writing.
Annie Dillard said a writer often has to give up the best piece of writing or take the book down the wrong fork. A comment that has intrigued me since I read it.