Characters. Without them, plot is meaningless. There would be no way to forge an emotional connection to what's going on. Not to mention no one would be talking to each other, and I don't think I can function in a world without dialogue.
The reasons for creating characters were easy enough to find. Actually creating them, forming people with histories and hobbies and feelings and flaws, is more difficult. They have to be compelling, they have to be realistic, they usually have to exhibit some sort of growth, and I suppose most of them should be somewhat likable.
So you've created a character, you've given him years of history that takes up several pages of your outlines, you know what he looks like right down to the last freckle on his chin, and you know how he'll react to every set of circumstances you're going to fling at him. Now what? We've reached a tricky spot again. We want our readers to form a bond with him, but we don't want to fall into the dreaded "info dump" trap. There are also cliches and tropes and stereotypes to avoid. Oh, and we have to work in some plot and conflict somewhere along the line. It's a wonder anything gets done!
If your hero is super strong and brave and seemingly invincible, readers will get annoyed if there's not something there to balance it out. If your heroine is whiny and sniveling and sitting around waiting to be rescued, readers will get annoyed if there's not something to balance it out. If whatever you choose to balance them out isn't convincing (like, you reveal 75% of the way through the book that your unsympathetic unwilling partner is always cranky because when he was two years old, he watched his mother get mauled to death by a bear, or something like that), what happens then? You guessed it! Readers will get annoyed! (And really, it seems like it doesn't take a whole lot to annoy readers. They're a picky lot.)
The really pesky thing is that there doesn't seem to be a formula for creating great characters. Or if there is, no one's discovered it yet. (Note to self: put that on the list of potential ways to make millions.) I think one of the most important things we can do is to take a step back and try to look at our creations objectively. I know, I know, I've heard it before - our characters are our children, we grow attached, blah blah blah. But just because we love them doesn't mean the rest of the world is going to automatically love them as well. We have to EARN that love!
(scampers off to figure out how....)