As I commute everyday by NYC subway, I have observed as well as personally experienced the frustrations from millions of subway riders swiping metro card at the turnstiles. According to MTA's answer to how to use metro card on the subway, "With the MetroCard name facing toward you, quickly swipe your MetroCard through the turnstile in one smooth move. Walk through when the turnstile screen says "GO." It sounds fairly intuitive and simple, get the direction correct, swipe and go, which aligns with my assumption of how to use the metro card in order to get into the station.
However, the reality through my observation is that, 4 out of 10 people don't get through the turnstile in one swipe. They have to try at least 2 to 3 times and sometimes over 5 times. It is very frustrating for not only the person trying to get through but also people who waiting behind during rush hours while there is only one turnstile at certain stops, if one person gets stuck, 10 others are stuck there waiting until he gets it right, while time is being wasted. Most of the time, I can simply tell whether the someone is new to the MTA subway system by the way they swipe their metro card. For regular riders like me, most of the time they can get through within 2 seconds; however for many tourists, they can get stuck there for as long as a minute straight helplessly trying to figure out what the problem is.
If you swipe too fast or too slow, or not smoothly enough, it will not work until you get the perfectly right speed. However, lots of new subway riders wouldn't realize where the problem is because it simply shows "Swipe card again at this turnstile". Sometimes they would change the direction of swiping, sometimes they would flip the card, and they still couldn't get it right. Sometimes it takes another rider to help the person swipe the card to get through the turnstile.
The interface is very simple with just a card reader and a screen that gives instructions. Therefore there is only one control - reading the card. However it takes many repeated actions of swiping to get it work, resulting in difficulties. On one hand, it is due to technical issues, the sensitivity/capability of the reader and the material of the metro card being too thin and slippery. On the other hand, it is the design issue as there is not enough visual clues to indicate what should be the correct way of swiping card, should I go faster, or slower ? There is not enough constraints to limit the possibilities to avoid confusion, the card can go in either direction and either side facing the rider, and it is not clearly indicated on either the card or the reader as how to correctly swipe the card.
In many other countries, subway systems have much more intuitive turnstile interfaces that eliminate all those possibilities that might cause confusions. Turnstiles of Shanghai subway, for example, has readers with the shape of the card and says "please tap card". This could solve all the problems from manually swiping the card, with a simple tap. It is much harder to get a tap wrong than a swipe.