The best touch games are often the simplest ones. Infinite Stairs only knows two inputs and one goal. All you do is climb as high as possible on a never-ending set of stairs. Tapping on the right side of the screen makes the character go one step up, whereas the many turns on the randomly generated staircase require tapping the left side (or vice versa, optionally). Coins collected on the way can be used to unlock extras. That's it.
This may sound stupid easy, but the controls are not that intuitive. Due to the step/turn dichotomy instead of a direct mapping to the left and right, it's easy to slip up. The turn is not just a turn in place, but always comes with a step in the new direction, which can get confusing at twisted segments.
But counter-intuitiveness is a good thing in this case and makes the game much more interesting. You need to hurry, lest you lose your balance, represented by an ever depleting gauge that is slightly recharged with every step. If you don't keep moving, you'll plummet down Looney Tunes style. The first 150 steps give ample time to think through every action, but the gauge goes down faster the higher you get, until there is no time to breathe at all. When the beginning gets too boring, you can skip the first 250 steps for 100 coins.
For motivation, Infinite Stairs has a lot of content to unlock. There are many playable characters, from the default businessman to a cheerleader, a plumber, a rapper, a nutcracker man, a teddy suit and even Zeus. Each requires a special condition to unlock, like reaching a certain height, accumulating a total amount of steps taken or playing the game on three Saturdays. Most can also be unlocked with coins, except for the best height ones, so you'll need almost inhuman reflexes to play as Dracula or the bunny girl. All characters play the same, but some have irritating animations.
Coins are also used for buying pets (cat, dog, pig, gorilla, parrot, potted plant, robot...). They do nothing but follow your every step and gather experience while climbing, their look changing slightly whenever they reach a new level. Unfortunately, they cannot revert to normal afterwards if you don't like the cat's tutu, for example.
Most expensive to unlock are the stages. Like characters and pets, they're purely cosmetic but feature fun (unofficial) cameos, like Faith from Mirror's Edge on the rooftops or Jar Jar Binks on a balcony of the space city. The backgrounds keep scrolling by slowly, and while they're completely static, new and interesting things keep appearing even after hundreds of steps.
Infinite Stairs is entirely driven by advertisement. The top of the screen is occupied by a banner and sometimes there's a full-screen ad after losing. You can also earn quick coins for testing other programs or watching trailers. Whenever you set a new record, the game bothers you about logging in to Facebook. You can always reject, but never get rid of the prompt. There is no ad-free premium version, either.
At least the advertisement model prevents abusive microtransactions from compromising the game's appeal. Infinite Stairs is fast and frantic with super short session length, where success increasingly depends on intuition built from previous tries, so the lack of obstructions for trying again and again is crucial in making it as addictive as it is.