Two examples:

1- Season 3 of the Fox television show, Bones.  I love this show.  It’s now in its fifth season and has a promise of season 6.  (Which I’m sad to say I hope is its final season.)  Season 3 was the ill-fated Writer’s Strike season.  Thus, the pacing was off quite a bit.  The finale episode climax was pressed into a single episode instead of the hoped-for arc of 2-3 episodes.  Thus many fans felt completely justified in being angry over the outcome of the hunt for a serial killer and his apprentice.  (Think Sith Lords, here, and you’ll be on track.  Heck, they even used that reference in an earlier season 3 episode when discussing this killer.)  The pacing was off because of the strike.

2- The season finale of the Sci Fi Channel original series, Warehouse 13.  There was a huge reveal in the final moments that came completely out of the blue.  The pacing and integration of the character whom the reveal was about was completely off.  In any work, when you throw a monkey wrench in the works like this, where a character you thought was good is really in cahoots with the villain, you need your viewers (and for us, readers) to be able to go back on the season (or chapters) and go, “Oh, I can totally see it.”  But this won’t happen if you rush it or if your character isn’t involved at all.  Your character must be either the hare, the tortoise, or the ref at the race’s finish line, but can’t be the random bug that might buzz the hare’s or tortoise’s head once in a while.

Pacing eludes me.  (Or should that be evades?)  I’m constantly struggling with it.  I have a tendency to rush through everything because I just want to get the words out on the page.  In 12th grade, the first time around on Oracles Promise (though it was titled Sabrina back then, I think) I had this problem.  My 12th grade English teacher was kind enough to take time from his busy schedule to read through my work.  A repeated comment I received from him was to elaborate on a scene.  So I would.  And often that scene would triple in page count.  Often, after elaborating on the scenes that he had suggested, my chapter would have doubled in length.

But pacing would still be a problem for me.

So I soldier on, and keep track of where I want to add more details.  Really I need to add more scene to my dialogue sections.  Again with the pacing.

Pacing makes everything believable or unbelievable.  It’s, in my opinion, the critical factor in any work.  Second only to characterization.  If you rush through, your readers aren’t going to fall in love with your characters like you have.  If you go too slow, they’re going to put down the book before ever reading the wonderful twists and turns that the climax will provide.

The moral here, though, is that your readers are smart and they’re going to know if you’ve rushed something or if you’re lingering too long on someting else.