I'm curious to see what various theories and ideas people have about the world of Equestria, facts not given by the creators but imagined to fit in the minds of the viewers based of information gathered from the show.
For example, I believe that, due to magical health care, ponies can live for hundreds of years. Why? Because in Winter Wrap Up, Twilight claims that "For hundreds of years, [Ponyville's] never used magic to clean up winter", but in Family Appreciation Day, we learn that Granny Smith was among the founders of Ponyville. Ergo, Granny Smith must be hundreds of years old, and we can assume it was thanks to magic being able to cure most ailments and injuries.
I'm also fond of the idea that the Flim Flam brothers are actually Changlings who go from town to town, trying to settle a new colony for the Hive. First off, their magic is green, like changeling magic. Second, their cutie marks are apples, but I'd say their special talents are more suited to swindling others, or even engineering (they built a car!), and they merely adopted the apple marks to convince the townsfolk of their "skill" in cider-making. Third, if they succeeded in kicking the Apple Family out and established themselves as Ponyville's chief apple-sellers, they'd begin to win the love and affection of the town, which would make it easier for the other changelings to come in and take the rest of it by force.
Those are my two pet theories, but I'd love to hear what others think, so feel free to include your own ideas on Equestria's inner workings, or criticize mine. They more ideas we have, the more detailed our map becomes!