"Strong writing requires an intimate knowledge of humanity."
~From Creating the Story
It's true: elaborate, detailed character development is important, but it won't get you anywhere without deep, familiar knowledge of human nature. A writer has to know and understand the workings of the human mind and apply their knowledge to their characters in order to effectively transport a reader into that character's thoughts and struggles. A reader cannot identify with a character on a deep level if that character's mind doesn't function like a realistic human mind.
So that leaves us, the writers, with the task of learning and studying human nature. You'd think it would be totally easy--after all, we're humans, aren't we? Of course we know how we think!
But it's not always that easy. We tend to take our human nature for granted; it's part of us, so we don't think about it that often, just like you probably take your right leg for granted. Have you ever just sat down and studied your right leg, the way the muscles fit together, the way the tendons flex and move when you bend your knee, the way the components of your knee joint work together? I'm going to venture a guess and say that most of us probably haven't. So have you ever sat down and analyzed your own thought processes and mental gear-grinding?
A warning: don't become over-analytical! You don't want your writing to become bogged down with the minute details of every character's thought processes, nor do you want to get so into the habit of analyzing yourself that you spend all of your time analyzing potential decisions rather than actually making them!
The microscopic details of characters' thought processes aren't what's important. What's important is the understanding of the character's nature that you bring to the page. Even if you don't openly display your understanding in a lengthy dissertation on why Character A made this particular choice, if you have that deep, intimate knowledge of human nature and the workings of the human mind, it will show. And if you do it well, the reader will benefit from it without even realizing that it's there.