Since I was as young a writing student as I can remember, I have been taught to cite my sources; I was instructed that you could use any material you wanted to strengthen your argument - in fact, the more material you had to substantiate whatever you were saying, the better - but citations were critical to staying legitimate, and that without citations, you were breaking the rules. The rules eventually became the law, and I was always under the impression that one could face penalties if they didn't cite their sources in professional work.
Now that I'm working on publishing my own material, I've found that I have been led astray. Citing your sources is nice - it's kind of like tipping the waitress after a good meal. But you can't use other work in your text - cited or otherwise - without explicit permission from the author or copyright holder. Fair use doesn't extend to commercial work, and while it is vague enough to seem like it would cover academic-style writing that is written for profit, I'm certainly not going to put my bank account on the line and get sued over a few quotes.
So I spent the day identifying the source of over a hundred quotes in my small book, finding the publisher and respective address for the permissions department, and writing letters to each one asking for permission to use their words. It was tedious, but now it's done. I suppose if they don't grant me permission, I'll need to find another way to say what I want to say, but doing something as menial as that sort of made the book seem all the more "real".
It also made me truly respect the creative commons style of ownership; I'm considering actually publishing my own work under that style of "copyright", allowing others to quote liberally.
I owe a mean spirited letter to my high school English teachers. Citing your sources. Sheesh.