Serious Business, also rendered SRS BSNS and other such variations, is a term almost always used in a facetious way to refer to things that are nothing of the sort, but that people blow out of proportion until everyone is ready to break out the flamethrowers. In other words, Serious Business is a term for the mountains which have been made out of molehills. It is always capitalized to show how serious the business is. And if you believe that last line, you haven't been paying attention.
Mary Sues are often guilty of Serious Business, especially in the form of gratuitous angst and drama over ridiculous things, such as being too beautiful. While this is not as serious flagrant an infraction against good sense as trivialization, it is still annoying and may be a charge if carried to extremes.
The PPC is emphatically NOT Serious Business. The PPC is well aware that it is silly, and likes it that way.
Trolls are strongly attracted to Serious Business and are all too willing to use the drama produced by it to their advantage.
Not to be confused with Sirius Business.