I think Deadpool was grandfathered in, so to speak. They tried and failed to make him fit a PG-13 rating, no one would go for it. So their hands were kind of tied and they relented to keeping it R.
That's of course just the impression I got from it all, but it's hard for me to believe they'll allow any other character to be R rated. I'm still skeptical on how they'll handle Deadpool's vulgarity.
I don't kill... but I don't lose either.