Since the posts about zombie science (zcience? Sorry) and ScienceMagic(TM), I've been thinking about when explaining a thing takes away from the impact of the story. See, in genre fiction, we deal with a lot of made up things. These things have to be explained to the audience to make sense. If you tell me Dan the Trader has a Zoofbhorn, I have no clue if that's a weapon, a horse, or a type of exotic candy cane. Often, these things can be explained in the background—if someone's sucking on a Zoofbhorn, I can reasonably rule out the weapon option.
This leads to genre authors making up tons of stuff and then explaining it to the audience. And that's great! Part of the draw of genre stories is how they spread our imagination—even if things aren't possible, perhaps one day they will be. That said, the reader's still expecting a story, so we need to explain how these made-up things interact with the world, and hopefully do it in a flavorful manner. Then we can get back to the story.
But sometimes creators overexplain their tropes. The classic example is midi-chlorians. Midi-chlorians were Lucas's way of sciencing his space psychics so they would seem plausible. Unfortunately, they raised more questions than they answered, provided no real story benefit, and shattered the mysticism of the original trilogy. I'm certain there are more reasons why midi-chlorians are a bad idea, but those are the main ones. (Okay, one more: There's an unnecessary dash in their name.)
It seems like those are three don'ts of explaining anything: Don't raise more questions; Don't break established flavor; and Don't explain for no reason. Generally, every explanation needs to be better than an old guy with a beard getting in the protagonist's face and saying "Just trust me, okay?" Because that's all most people need to kick in their suspension of disbelief.
That said, what do you think was overexplained? What have I missed?