There are three problems with providing too much physical description of characters (and to some extent, of settings).
First, timing. Beginning authors often feel they have to provide the character's appearance immediately upon that character's initial entrance. There is certainly a logic to that, but then what one often gets is expository lump right in the middle of what is supposed to be an action scene. Say an assassin jumps out at our hero: if the author feels compelled to provide a detailed description of what the killer looks like, then instead of the rapid pace of swordplay, gun fire or fisticuffs, the story comes to a complete standstill while we are briefed on disheveled hair, wild eyes, rumpled suit, and so on down to the shoelaces. Interesting as all of this might be, it is less relevant and compelling then the fact the individual in question is trying to kill the viewpoint character.
When police try to debrief an incident, for example, the witnesses are often hard pressed to identify their assailant's hair and eye color and height and so on because their attention was pretty much focused on the fact that they were being assaulted. In the heat of the moment, eye color is pretty far down the list of what people notice; so the reader won't really notice its absence either, if the writer provides sufficient action. What the reader will notice is that description replaced action; that the action ground to an unexpected halt at the precise moment the author should have been building tension.
Second, although the author may have cast the character in a particular way, imposing that one specific actor/description on the reader is unnecessarily restrictive. Yes, the author may have worked hard to picture the scene s/he is trying to depict down to the specifics of hair and eye color, but contrary to the beginner's understanding of the process, the writer's job is not to reproduce that scene in the reader's brain exactly as the author originally pictured it. On the contrary, one wants a certain level of vagueness, of blank canvas, onto which the reader may project their own experiences and preferences. Just as a playwright has to allow for a certain amount of interpretation of the script by the director and actors, the writer has to leave room for the reader to bring something to the project.
For example, if the story features a bully, then it is far better if in the reader's mind that bully merges with that bastard down in accounting who is currently making their life miserable. Of course their conscious mind is not about to suffer any such confusion, since pretty sure guy in accounting is not in fact king of the space vampires, or whatever; but great fiction, like great opera, often bypasses the intellect and goes directly to the viscera, with people's emotions. The resonance between the writing and the reader's own experience may be disrupted, however, if one insists on establishing definitively that the guy in accounting is not the bully under discussion because the one in the book has red hair and blue eyes.
Or, to take an example from the other end of the emotional scale, if one is too precise in describing the love interest, one runs the risk of including a detail that is, for the reader, a deal breaker. "Electric blue eyes" are as likely to remind them of their ex as of their current lover. (It is the same reason why it seldom pays to be too explicit in sex scenes: if it doesn't happen to be the reader's kink, one is more likely to get an "eewww!" than a sale.)
So why go there? If the writer insists on determining every microscopic detail of the experience for the reader because that happened to be how the writer pictured the scene, then it's not about trying to be precise, it's about being a control freak. If one wants to build readership, one has to give up some control so the reader can take some ownership of the reading experience. If one wants readers to recommend the book to their friends, then the reader has to come to think of it as one of their books.
Third, the author of course believes one breathes life into a character by providing all this detail; but in fact it often has the opposite effect: by lavishing attention on the physical description, the author is to that same degree likely to skimp on actual characterization. Eye color does not a character make, because one can randomly (re)assign hair and eye color and not change the character in any fundamental way. (Well, unless these things have special significance in this particular SF&F world, that grey eyes indicates elvish ancestry or some such...). Characters are generally memorable because of their actions, motivations, attitudes, strengths, flaws—in short their personalities— rather than eye or hair color. If one's character notes are all about physical appearance, then you're doing it wrong.
As we frequently reassure each other, it's not appearance that counts, but what's inside.
Which is not to suggest that one should never provide any detail of appearance or setting; only that one needs to ensure these details are inserted when timely and relevant; that they don't occur as a disruption of the narrative, or in overwhelming quantity (see previous column, "Common Mistakes #2: Less is More).
[Cartoon stolen from Bliss by Harry Bliss Sept 22, 2015)