In case you're wondering whether this is a big deal at all, the answer is yes. This is something like Bill Clinton doing his forum on MTV, only instead of dumb questions about his choice of underwear, the president took some interesting questions about issues that matter to people. I hope Reddit AMAs become a regular part of campaigning. You can still ignore the questions you want to ignore, but Reddit's upvote system provides a pretty good sense of when users feel like a question or answer is bullshit. Go read the exchanges.
I do have to say I'm a little disappointed that the president, who at one time considered himself to be a writer first and foremost, doesn't capitalize proper nouns and has their/they're/there issues. But this was a good thing to do. It's a minor piece of campaigning history that may well influence the future of campaigns for the better.