I think all of us must have written mirror scenes at one time or another. When I was earlier on in my development as a writer, I was far more visual - far more movielike, in a sense, because I really wanted to be able to imagine my characters in detail. I took the time to draw them to the best of my ability, and I thought about how to describe them based on that.
That can work fine so long as you are observing your characters from the outside - either as a character who observes them, or as an omniscient or distant narrator.
However, the further I've gone into writing, the more interested I've become in portraying my characters from the inside, and I quickly realized that if you are using deep point of view, and you put a lot of attention on a character's appearance, it automatically implies (via "show-don't-tell") that the character is vain and puts a lot of attention on his/her own appearance. In other words, put a person in front of a mirror and they will look like a person who spends significant amounts of time in front of a mirror. Put them there and let them comment on the flowing quality of their own dark locks, their own silky skin, or the depths of their own limpid eyes, and it starts to get seriously, narcissistically weird.
If you're going to put someone in front of a mirror, have a good reason to do so. In For Love, For Power, my main protagonist, Tagaret, never stands in front of a mirror. Not even to check if he's buttoned his cuffs properly (he asks a servant to check). His brother Nekantor stands in front of mirrors occasionally, but he's obsessive about having a perfect appearance, and he also finds mirrors very comforting because they're smooth and cold. For him a mirror is a way to keep from freaking out. When I put my third character, Imbati Aloran, in front of a mirror, it's to point out something pretty important:
...to appear at an interview unmarked would be to fail before he began. He went to the mirror he shared with his bunkmate and painted the small black circle between his eyebrows. Then he combed his dark hair into its ponytail, which thanks to Kiit's precise trimming, fell just outside his collar. He shut both makeup brush and comb back into his box of implements.
Previously in the story, we've seen members of the servant caste with tattoos on their foreheads; here, Aloran is showing us that young people of the caste don't have such tattoos, but must paint a mark on each day. He also takes care of his hair - but for him, personal appearance is a professional concern, not a personal one.
Generally speaking, if I think it's really critical to get an element of appearance in somewhere, I try to sneak it in. That's why I put "his dark hair" in the above quote, rather than just "his hair." One little extra adjective is enough, because the other elements of the description are more important.
Readers can generally fill in a lot of appearance details out of their own imaginations, just on the basis of the personality and behavior of a character. This is pretty amazing; it's also one of the reasons why I'd encourage you to be up front with important appearance details that you don't want readers to guess wrong.
If you're working with a group of people with homogeneous appearance, you need to be cautious and deliberate. I have written stories set in Japan; a friend of mine is working in a fantasy world where everyone has brown skin and black hair, with variations thereupon; the same caution would apply to a world where everyone was pale. What makes this tricky is putting the basic characteristics out there so your reader won't make an incorrect guess, and at the same time establishing the standard appearance without making it stick out as somehow unusual (from an internal point of view, it's not unusual at all). Over-description will make it seem unusual and give a sense of undue concern with appearance; under-description has other pitfalls. Put yourself in the position of the protagonist, and ask, "What does stand out?" If all hair is light, then will a particular style be remarkable? If all hair is dark, will a particular type of hair ornament be what stands out? Put the focus on what stands out to the insider-observer.
I'd like to issue a similar caution for those working with cultural differences. A "cultural mirror scene" would be one where the character is called upon to declare, "I am a X and because I am a X, I value this and I behave like this." Real situations where people have to do this are actually uncommon. They usually arise in conflict (politics on Facebook comes to mind, where people are anonymous yet called upon to take sides). In another type of cultural mirror scene, a character has to come face to face with an outsider to the culture, and take a big (figurative) step back in order to look at both conflicting cultures from a distant point of view, observing the difference between the two ways. You can also create situations where children are being taught, because those give adults a good opportunity to say, "We are this, and as this, we are supposed to behave like this."
More often, a person will notice the cultural behaviors and manners of others, rather than his/her own behaviors. Often you can just dial it back by a step - have a person remark on another person's culturally based behavior, but not include an explicit comparison with their own. Very often, the observer is only aware of the other's behavior as behavior, and not as culturally influenced behavior. As I see it, we're trying to avoid writing our characters as anthropologist/ethnographers unless they actually are anthropologist/ethnographers.
Try to be very sensitive to when and why you are putting "mirrors" around - and by that I mean whenever you have someone explicitly notice a contrast of manners or cultural behavior. In the same way that you don't want to have a character walking through a hall of mirrors and noticing another detail of their appearance every two or three steps, you don't want to exhaust your reader's cultural sensitivity by having your point of view character be hyper-aware of every last cultural contrast or detail in the surrounding environment. Make sure that when you use them, you are using them to demonstrate something important to the character, and to the story.
It's something to think about.