I think that my favorite thing that authors do in writing is foreshadow. A good use of foreshadow will make me fall in love with an author like nothing else.
To be honest with you, my adoration of JK Rowling started with foreshadow--she layered in the Polyjuice Potion that was crucial to Book 4 in Book 2, and by that point, I was just head-over-heels. Guys, good foreshadowing is just smexy.
Foreshadow takes skill. It requires one of two things--either the author has to plan out the important clues from the beginning (especially difficult if you're dealing with a series instead of just one book) or the author has to use the situations/objects/characters she's already established in the story in a significant way later on.
And either way you do it, you've got to make it clever.
In a lot of ways, foreshadowing is like a magic trick. Magic are all about diversion. I make you look at the flash and bang over here and you don't notice the card up my sleeve over there. Likewise, (good) foreshadow is all about showing you something in plain sight over here, but hiding its significance over there.
Foreshadow is essential--essential--to a good story.
But bad foreshadowing? Bad foreshadowing can kill a good story.
Doctor Who Spoiler Warning Below This Point
For example: y'all know I have some love for Doctor Who, right? Well, last night I watched the latest episode "Let's Kill Hitler" (and if you've not seen it, stop reading now to avoid spoilers). And...as much as I love Doctor Who, I have to say...the entire storyline is weaker to me now because it didn't use good foreshadow. In fact, it didn't use any...and it totally should have. Here's the thing: it tells the story of Mels, Amy and Rory's childhood friend who is (a) obsessed with the Doctor and (b) brings him up all the time and (c) is the person Amy and Rory named their daughter after. And...she has never EVER been brought up on the show AT ALL. There's a complicated plot twist about Mels that you can read about here, but the thing is--I didn't care at all.
Do you know how cool it would have been if, when we saw Amy as a child last year, we also saw childhood Mel? How about if Mel had been at Amy and Rory's wedding (if she was significant enough to be the namesake of their child, why not have her at the wedding)? How about if--in the last freaking episode--Amy had mentioned that her daughter was named after her childhood friend?
But no--none of that was there. Which made the entire story not very believable. Which drove me mad. Which meant that this story of Doctor Who--which I've loved so far, full of time travel and witty dialog and clever plot twists--is diminished in my eyes.
That is the power of foreshadow.