Matthew J.X. Malady in Slate:
2. One change that should have been made to the keyboard decades ago is the addition of a dedicated em-dash key. An em-dash is meant to indicate an abrupt change of thought within the context of a sentence. Writers of all stripes use them often—sometimes too often—but they can be a real pain in the carpal to type.
To make an em-dash using a Mac, you have to do this: First, press the option key. Next, while holding down “option,” press “shift.” Now, while keeping those other two buttons pressed, hit the hyphen key. It’s too much—three keys for one mark. On a PC, there’s a handy “shortcut.” Simply hold down “alt” and then type 0151 on the far right number pad. (Next challenge: safecracking.) Although some popular word processing programs will automatically create an em-dash when you type two consecutive hyphens, that’s no reason to prolong the mark’s banishment from the board.
(At least partially because there’s no dedicated em-dash button on the keyboard, people mess up this mark in many annoying ways. Some use two hyphens–like so. It’s not an attractive replacement. Other typists resort to a single hyphen as a stand-in for an em-dash-like so. That’s just confusing.)
More here. [I heartily endorse an em-dash key. I usually have to copy and paste it from somewhere.]