It depends on who's doing the describing.
Some objective, omniscient narrator? Describe features objectively. Focus especially on anything that's relevant to the story (e.g., a physical attribute that becomes a plot point later on), anything that reveals their role in the story (e.g., a uniform), anything that shapes the character's self identity (e.g., insecure because of a lazy eye), or anything that affects other characters' perceptions of them (e.g., a slumped posture).
But if your narrator is another character (or if you're using a close third-person where you're in a character's head), describe the details the narrator would notice. This gives insight both into who the new character is and into the narrator's world view. Maybe an assassin sizes up everyone she meets in terms of whether they might present a threat. A womanizer describing a woman is going to focus on aspects of her physical appeal and take educated guesses about her psychological vulnerabilities. A high school teacher might not pay much attention to whether a student is a sophomore or a junior, but the students likely would apply these labels to each other.
In general, use some very specific detail that makes each character different from all the others. The reader can latch onto that as a label to remember who's who and fill in a lot of the irrelevant details. These labels should be the features that dominate our impressions of these characters.
Traits don't just have to be visual. Smells, sounds, and textures can also be interesting features of your characters. As can mannerisms, tics, accent, posture, etc.
Do not assume some standard lump of human and then describe only the attributes that make them different from that standard, especially if that standard is yourself. For example, if you mention the skin color only of black people or the gender of only the female characters or the age of only the elderly ones, you risk coming across racist, sexist, and ageist. (Of course, if you're in the head of a racist, sexist, ageist character, then it might be appropriate.)
Robert Sawyer is well known for including all these details for all his characters, in order to create a diverse, interesting, and fairly treated cast. The readers aren't left to assume the skin color of the new character is like their own.
At the other extreme, you could never mention these details for anybody. But then you can't write a story about race or gender or age, or even one where those even factor in. It might also seem a bit generic to readers. People do notice skin color, gender, and age. If your story doesn't, it might come across as rather bland.
A character's race, sex, and age should not be the defining trait that makes them stand out from the rest of the cast unless it's central to the story: the one woman in the boardroom, the one Hispanic left in the gentrified neighborhood, the one employee with decades of experience in the new startup, the boy in the wheelchair who takes the elevator alone while his buddies race down the stairs.