You are deep into a mission in a game to infiltrate a building. There are hostiles all over the place but luckily you are a skilled military operative. You approach a door, only to find it locked. You try another door, locked as well. Can't climb the pillar? Can't jump over the balcony? You spend precious time trying to figure out what to do. Time ticks by.
Soon you don't feel like a bad-ass operative, just another lost gamer. Instead of being immersed in the experience it feels like a game of "read the designer's mind." At Monolith, they try to make sure players are having fun with the game and avoid situations like this. In order to do this, they run extensive playtests on all of their games.