This one really resonated with me. The first book I wrote, I made a conscious effort to include as little physical description of the main character as possible: I wanted readers to be able to project themselves into that heroine role. Personally, I dislike a lot of physical description of characters in a story--I always imagine myself in the story, and too much physical description threw me out of the story.
“Many writers express the character’s backstory before they get to the plot. Good writers will go back and cut that stuff out and get right to the plot. The character’s backstory stays with them—it’s in their DNA.
“To paraphrase Bruno Bettelheim: ‘The more the character in a fairy tale is described, the less the audience will identify with him. … The less the character is characterized and described, the more likely the reader is to identify with him.’ ”
—Adam Chromy, Artists and Artisans
However, all my beta readers wanted to know more about what the main character looked like. It bothered them that I hadn't included that information.
There's a fine line there. Think of Harry Potter: we all know what he looked like (even before Daniel Radcliffe), and that book was not one that threw me out of the story, despite the fact that it was a skinny 12 year old as the narrator.
What it comes down to is this: relevance. If the details are relevant, then there's a purpose to it; it becomes part of the story. Harry Potter's physical description is often describe as messy black hair, jade green eyes, and the scar. Each of these things is important to the character development and the plot.
Now, I know that physical description isn't the same thing as background, but it is a part of it. And, as with physical description: relevance is the key.