See Fun and Challenge.
In the past few decades, as game design has become a discipline and generally been tightened up, games (other than pure strategy games) have naturally gravitated toward elements that are both fun and challenging.
- Taboo and many other PartyGames? include a sand timer. Each turn lasts a fixed amount of time, in which a team can score (theoretically) any number of points--maybe none, maybe two, maybe seven. Of itself, the timer is a challenge: it's a constraint that shapes the nature of the competition. But it also contributes to fun by limiting the amount of time that can be spent on any one round, by encouraging Speed?, and by both encouraging BurstsOfActivity? and highlighting them when they occur ("seven points--wow, what a turn!").
- (Other examples?)