I think the biggest Overton shift this cycle is that anybody thinks they're qualified to be president, and the electorate seems not to mind. Perhaps we should blame Obama, who came in as a one-term Senator.
Look at the current GOP field, in order of current poll rank: (1) real estate developer, reality show star, and lifestyle brand pitchman who has never held public office; (2) former state solicitor general halfway through freshman Senate term; (3) former state senator nearly through freshman Senate term; (4) brain surgeon who has never held public office; (5) former governor and current US Representative.
I'm probably forgetting a number of also-rans in past cycles, but my feeling is that in the past, only (5) and maybe (3) would feel they had any business running for president. I think we're entering a world, driven by cheap and easy information and publicity, in which any yahoo thinks they can run for the highest office and build credible support.