To be honest, I've always wondered what an alien would find most alarming or strange about humans at first sight. Simply getting accustomed to things like buildings, cars, and public transport would be quite a task, but when an alien sat down in a public place and observed humans interacting and going about their day to day duties, I wonder what the strangest thing about us would be.

My guess - fashion and clothing. It would be easy for an alien to realize that the most basic function of clothing would be to accomodate the climate and environment, but guessing why everyone wears different styles and colors would take a while. Why do some people wear shorts , t-shirts, and sandals on the same day while others are in long black pants and polisehd shoes, with a collared shirt and bow-tie? Perhaps they are of a different class or have different functions in their society? Why do some people sitting at the same table wear denim-jeans and hoodies, while others wear hippie-pants, have their hair tied up, and have a single earring - perhaps they want to express themselves as different types of people? But why then did they make the specific choice to wear those types of clothing - are they aligned with a faction? Did they put on a random permutations of items? Do they wear the same style every day, or do they have a weekly set of outfits? For an alien to try and coneptualize the thousands of cultures, ideologies, social norms, and other factors that have resulted in our very diverse and cosmopolitan society (without the established contexts that we all grow up learning about) would be almost impossible in a few minutes.

It must be pretty strange to sit alone in a swirl of colors and objects, that all have directions unknown to the observer. I wouldn't know where to start.