There's the distinct potential that the general audience, expecting Deadpool to be a superhero film in line with the Marvel brand and the oh so common "from the company that brought you theX-Men Series" to walk out with their wee little ones highly disappointed and offended by the graphic language, violence and material.
The second cut could come from the extreme fanboy side that will leave the film jaded by the R-rating, having wanted even more profane behavior from the Merc with a mouth. There's a possibility that in wanting more, say an NC-17 rating (the stand in from the old X rating), those fans avoid the blockbuster generating multiple viewing. It could be a scenario of "I'll wait to see the unrated version on blu-ray."
So, while I'm all in on the R-rating, it does make me ponder on the side effects of it. Could it fracture the general audience and their perception of the mutant superhero universe FOX is creating ("They used to make good wholesome films I could take the kids to, but now I don't trust that")? For the general audience trusting a franchise is a big deal. And could other superhero film makers see push for more and amp up their properties ("If Deadpool can do it, so can we"). Which may again alienate the core profit generating audience.
Just throwing it out there.