Micromanagement. You build up a network of colonies and have to ensure that your people are happy, that all the goods you and they need are available in satisfying numbers, and of course that you gain more money from taxes than you lose from entertaining your industries. At later points in a campaign you might also be forced to wage war against the 4 CPU (or online) rivals, or even pirates.
In other words, you build up a colonial empire and have to manage EVERYTHING. While nice music plays, including stuff like Greensleaves.