In my experience, "Fun once, fun always" is the real key to a game that continues to be fun (and therefore keeps players). In a game about shooting virtual gun

In my experience, "Fun once, fun always" is the real key to a game that continues to be fun (and therefore keeps players). In a game about shooting virtual guns or driving virtual cars or building virtual cities, those basic actions had better continue to be enjoyable; no amount of rewards layered on top will fix a game with a poor basic mechanic. (As a side note, my first thought when reading "Rolling physical goods" was "Donkey Kong".)