One of the core features of computer games besides the graphics, sounds and story is something you don’t notice immediately. Some games do not do it very well but some became famous for it: Game Artificial Intelligence.
From the humble beginnings in games like Pacman to the great successes we know today like the Halo series, Game AI showed generations of kids that a computer can be pretty smart and sometimes even mean. Some of the better computer games adapt to the way the player reacts and then find new ways to compete. The aim is of course to keep the player interested in the game and engaged in the sense to make it just as difficult to challenge the player’s skills but on the other hand not to make it too frustrating or impossible to win.
Another part of good game design is that the controls are self explanatory and most gamers won’t be bothered with reading a manual before starting the game. If something is boring and thus means the player understood a strategy or principle of the game there needs to be a way to skip it and move on.