Let me join the sixty gazillion people who will have something to say about the news from the US Education Department. I'll make my comments in the form of predictions.
Duncan will spend his last couple of months doing what he's been doing for years-- making statements that sound pretty good, connecting them to terrible policies, and generally being ignored by Congress.
Some people will be really excited that he is leaving. These will be the same people who felt certain that Duncan was some sort of rogue agent, sneaking off to implement terrible policies behind the President's back, instead of the President's Guy, a cabinet bro who did just what Obama wanted him to. There have been fewer and fewer of these people over the years, but they still exist, and they still think that when Duncan leaves USED, things will get better. These folks should try investing in Floridian swampland.
Everyone east of the Mississippi who has been paying attention will express horror that former New York state ed chief John King had been names Duncan's replacement. In terms of policy, these two are cut from the same cloth. King loves him some Big Standardized Tests, test-based teacher evaluations, Common Core, and a whole world of privatization via barely-regulated charters. King has a hell of a personal story, from which he has failed to learn some critical lessons (a teacher changed his life, and he would like to install a system that would strap such a teacher to a giant steamrolling lemon). King does not suffer from a great deal of humility (he once astonishingly invoked both Martin Luther King, Jr., and Abraham Lincoln as supporters of Common Core and other reformy nonsense).
John King will make people actually miss Arne Duncan. King has a deft touch with the public. His biggest PR coup was to set up a series of meetings to educate the public about the awesomeness of his programs, and when they refused to sit quietly and be hectored, but instead actually talked back to him, King simply canceled the meetings. King never met a corporate reformy idea he didn't like, or a person objecting to such ideas that he thought was worth listening to. Duncan at least knew how to pay lip service to the concerns of stakeholders in public education; King will be so dismissive and obnoxious that folks will beg for Duncan to come back and tell us all pretty, pretty lies.
John King will most likely take office. There has been a lot of hopeful speculation that since he never went before Congress to take his "special advisor" position with USED, he somehow won't be eligible to act as secretary of the department. I can't think of any reason that would be true. And since he is a huge proponent of the reformy ideas beloved by republicans and democrats alike, I can't imagine who's going to stop him. Yes, he'll be terrible at it, but you may have noticed that competence is not necessary for gummint work.
John King will give a boost to national opt out. Remember, this is the guy who sowed the seeds of New York's opt out movement by being an intransigent asshat. There's no reason to think he won't scale that behavior up to the national level, allowing NY's highly successful opt outers to point and say, "See? See what we're talking about??"
King will also help convince Congress that any finished version of ESEA should keep the USED Secretary No Touching Anything clause.
Arne Duncan will get a nice cushy job somewhere, most likely working as a lobbyist or consultant or corporate hood ornament. He'll occasionally make a speech or a post somewhere, but nobody will be heavily influenced. If he is patient enough long enough, his stock will rise steadily as people forget what a disaster he was while they simultaneously build a huge storehouse of anger in reaction to Secretary King.
Fun times ahead, indeed.