One thing I've learned recently is that, when you're writing a story, you have to question every single word you put in it, every character's action, every piece of exposition. Everything has to make sense. Just because you can create people and plots with your words doesn't mean people have to believe them.
Several people told me they didn't "buy" pieces of my first story. I didn't understand why they were questioning my characters' motives...the characters just did what they did because. But now I'm realizing...because what? Because I, their all-powerful creator, said so? Characters need to be treated as real people, not puppets on strings. Now when I write, I question everything I make a character do. Why would he do this? Does this really make sense? Would this happen in real life? Is this how she would react?
I'm working on a new story for fiction, and I've already rewritten it once. Draft One was nine pages of, as one of my peer editors put it, character development, but nothing really happened. After my peer editing meeting, I suddenly had an idea that involved one of my characters from Draft One. I sat down at a computer and had a brand new story within twenty-four hours. My first draft, though I liked it, just couldn't compare, so I scrapped it.
That's the second lesson I've learned: don't become too attached to any story, character, detail...it may have to be cut in order to better your writing. These two lessons have already made me a better writer, and the semester isn't over yet!