For me, it tends to be what stands out most. I look at whatever is different--not on purpose. It just happens that way.

Their smile. Or more so do they laugh and appear relaxed.

After reading comments, I'd change my response. I said eyes, but I only notice the eyes of a woman, and only then after general stature (as someone said)/body type (I'm somewhat fitness-minded), gender, dress, and race. All of that is first level, impulse stuff.

Their body, usually their stature clothes and figure are far more telling than their eyes or smile.

Very interesting to see family groups together, see which family members have inherited the parents genes, hair colour, walking style etc etc

What I notice can be different for every person as everyone's unique. So, depends.

Face, body, posture, demeanor. In that order

Generally their overall figure, how they present themselves to the rest of the world, then the more telling things like the eyes

Eyes tell me everything about a person. They are truly the windows to the soul.......

There body language if you have ever been a victim of abuse or trained in self defense you learn to do this.

All of the above and within 10 seconds of seeing them.

Race and overall body type (fitness level, physical proportions) and how they are dressed.
How they move, gestures, then facial expressions. If everything looks okay, they seem relaxed and friendly i will make an eye contact... Voice is a big one for me.

I notice whether or not they're armed.

I notice where they look. How often they look.

Facial expressions when they see one person or another, what they might be saying based on gestures and levels of animation.

I notice where they walk. How they move. I look to see the easiest way to break that person in to a dozen little pieces with a fast, sudden set of attacks to knees, throat, elbows and shoulders.

I notice if they're moving with anyone else. From anyone else. To anyone else.

Sometimes I watch. Sometimes I pass over.

Skin color and features don't matter so much when you see all of humanity as a predator so long that it's all you care to notice.

Fortunately, for me and for them, I've realized that there's something besides their looks, clothes or smell. Something that shouldn't be messed with because just maybe they've had a bad day and don't need someone watching them like they're cattle to be processed.

People are neat. Well worth reserving any judgement on any of them until you get to know just what kinda crazy they are. 'cuz we're all crazy.

For me it's definitely body language.

I wanted to add a tag but guess I don't have enough points or there is something I am missing. Anyway the first thing I notice is gender.

Eyes, body language and the way that they relate to those around them.

Really depends on the presentation. If it is a direct face to face introduction, it would be the body language, then face / eyes.

Energy

Posture

Confidence shows in posture, Politics shows in posture (believe it or not), faith, happiness, hope ... we radiate the message with body long before the face. Eyes tell the soul Posture tells the person

I Guess a person right 99.9% of the time - Professional liars, Newsreaders and politicians are trained in posture, so they are harder to read.

Their eyes, they tell you what need to know, having worked with the public from being a Bar Manager to a Side Show Worker. The eyes tell you if a person is lying or truthful, if they have good intentions or not. Works for me.

Their voice...

Their sex.

Their mannerism, their body language and the amount of eye-contact they display.....these are good pointers towards someones level of intellect, and their Sincerity and honesty - and their current level of perceptive focus.

The eyes are the windows of the soul. You can tell a lot about a person just by looking in their eyes. (The next thing I check out is their butt. LoL!)