Games that play really well and stay fun for long periods of time do so because they have at least one strong dynamic. A dynamic is the back-and-forth pattern that tends to result in significant achievements on the way to winning a task, a scenario or even a whole campaign. Another way to describe a dynamic is as a pressurised grouping of loops that tends toward key decisions.
Game dynamics are hard to describe because they exist in between actions and overall tasks, and can only be sensed through repeated play testing. Like the hidden rules of rhyme or composition, game dynamics have no exact formula and so often seem to result in exciting, or even thaumatic, moments by accident.
In some games they are more apparent: In Rugby Union the main dynamic is the series of attacking passes and rucks that pushes toward the opponent’s try line. In Chess it is apparent in the tendency of pieces to form complex webs of covered territory, which then break apart after a decisive move. In Grand Theft Auto it is seen in the driving experience. In FarmVille it’s in the balance of timed growth and harvesting, leading to rewards. In Poker it is the process of betting and folding.
Fun comes from mastering game dynamics, so a great game dynamic is key to making a great game. Unfortunately it’s also very hard to do.