|The "dirty-public-restroom selfie"|
It usually goes something like this:
Amanda strolled through the hall with a hop in her step, turning the corner and catching sight of herself in the mirror: her long strawberry blonde hair falling down across her thin shoulders, light freckles sprinkled across her pale face and framing her fiery honey eyes.
This is a moment in the prose borne only to familiarize the reader with how the character looks; a chance to list out his or her physical traits in a quick and compact fashion so as to get on with the great business of storytelling.
And mirrors offer up such temptation, for many of us see them everyday in our lives, when we prepare ourselves for work and school and going out, whenever we go to the bathroom, whenever we are feeling unsure of our appearance. It's easy enough to simply have our characters nonchalantly stroll by a conveniently-placed mirror, take a quick momentary glance, and commentate on how she or he looks (as many of us do) at that moment. Because, what reader would care about our characters if they couldn't know exactly what they looked like?
The only practice more glaring than the dreaded mirror gaze is the act of listing. That most awkward act of stopping the story without any pretense to tell the reader all about the hairs on your main character's arms or how skinny their legs look while wearing shorts for no reason other than to say it - to give the reader "an image" of how the character should look. And usually, this type of stopping and listing will continue throughout the length of the story anytime someone new joins the party, either through the narrator's or the main character's eyes.
Derrick was a small boy, always had been, with knobby knees and a head of black hair that was too big and wild for his comparatively tiny head, obscuring his beady sky-blue eyes. Shorter than most boys his age, standing always at least three or four inches shy of his peers, the boy often felt himself to be a runt, almost always shunned by those who were taller and more athletic.
Unlike the mirror gaze, which at least attempts to fold this type of exposition into the forward motion of the story, the act of listing just flat out halts the action for the sake of throwing (most likely) useless and boring information at the reader. It's lazy, and adds to the notion that the reader must be told what to think about a character upfront rather than be trusted to come to their own decisions and conclusions organically as the tale proceeds.
Something about beholders and their eyes...
Look, no one expects your character to exist as an ambiguous, colorless, amorphous blobs of flesh ready to bend to whatever image the reader holds for them, but the fact remains that the reader will indeed imprint their own visions onto your creations, no matter how clearly you spell it out. That being said, aside from the very important task of giving characters personality, you should really have a detailed idea as to how they look like - the key though is to keep most of this information to yourself and to know when and where to dole it out.
Though readers will ultimately design the world of your story inside their minds, it is up to you, the writer, to lay out a good, strong foundation for them - not a blueprint. You are, after all, telling a story, not constructing a painstakingly-detailed diorama.
When a piece of scenery is described or a character's hair color revealed, it should always be in the context of the action taking place, it should be important to what's going on in that moment or be relevant to shaping the outlook of the characters doing the describing.
Seriously, stop with the itemization
Many times, I would argue, it is less important that a character look a specific way than he or she having a definite stature or demeanor within the story - or, a finely developed "essence" is more essential than knowing if the character has pierced ears or not.
Because frankly, these laundry lists of genetic features and social practices that you trot out at the beginning of your stories for your main character or whenever a new character first enters the narrative, are usually skimmed over or forgotten soon after they're read. No one wants to keep track of dry information when attempting to fall into the world of the story.
It simply stops dead the flow of the fiction in a most unnatural way.