Visibility of system status is an especially important design heuristic, as it serves as the fundamental process through which the product creator can give feedback to and dictate user/interface interaction.  The Mac OSX dock, shown below, exemplifies this principle well.

Upon turning on an Apple computer, users tend to instinctively move the mouse towards the Dock, given that it is one of the prominent features on the Mac Desktop.  The dock responds to the user hovering the mouse over one of the icons to facilitate the interaction: the name of the app will appear, and the icon will grow in size, which is helpful for those who can’t see as well or might have applications with similar icons (like iMessage vs. WeChat). In addition to showing the system status, the dock still preserves a sleek and minimalist style; in contrast to the Windows desktop, in which all of the names are displayed (and some of which go beyond the text limit and so aren’t fully visible), there is no excess text on the dock.  Meanwhile, the basic switch, pictured below, violates this visibility principle.

As you can see, the switch has a very minimalist design; in this case, too minimalist.  It gives no indication of whether the system is on or off, unless you could see the device (i.e. a light) it was connected to. For instance, often times if a bulb is conked out, users tend to flick the switch multiple times to check that the system is set to on.  It is surprising that this flaw continues to persist despite the simplicity of the fix.  The fix could even be so easy as to writing “on” or “off” on the side of the switch facing the user, like below:

More complex solutions could include additional features, such as determining whether the system is out of power/broken (as in the case of the light conked out).