It starts with a wide collection of highly customizable complications, which range in function from date, to weather, to astronomy. Each can be adjusted precisely to best fit your desired function and appearance.
This set of complications can then be dynamically scheduled to appear on your watch face following rules you define. For example, a particular complication slot could show the weather first thing in the morning, then your calendar during your work day, then switch to your Activity ring progress as you wrap up your day. This lets you take full advantage of each slot on your watch face without the need to constantly switch back and forth between faces… More here.
All we need now is for Apple to offer some decent customisations for the Apple Watch.