Among writing academics are partisans of the colon and those of the dash, as well as neutrals. Often the criterion is register—the colon designated formal, the single dash informal—but formality doesn’t necessarily recommend usage. Legal-writing authority Professor John R. Trimble takes a distinctive position, favoring the dash over conjunctive colons because colons look overly formal (“studied”). Trimble may have over-generalized from the correct observation that the colon is overkill when the matter’s explanatory character is obvious without it, as in this sentence:
There are two parties to a sales contract—buyer and seller.A colon would induce excessive expectations.
Another warrant for the dash in the last (italicized) example sentence is that emphasis doesn’t fall on the explanatory matter following. The colon emphasizes what follows, a pair of dashes what they enclose, but the single dash emphasizes what precedes, an emphasis writers can exploit to offset the dramatic character of what follows. This effect can trick an observer into concluding that the dash, not the meaning of what follows it, provides terminal emphasis, as here:
Employing a single em dash in a sentence commands your readers' attention, enticing them forward—c'mon, reader, let's go see what'z over here! It can also lend particular force to a terminal phrase—really it will!
Hamlet's indecisiveness, his arrogance, his suspicion of others, his passionate, brooding, introspective nature—these all contribute to his downfall.