The movie Ten Things I Hate About You can make anyone well-versed (snicker) in literature groan, because it's just another re-hashing of a Shakespeare plot (in this case, The Taming of The Shrew). West Side Story is copied off Romeo and Juliet. There are countless books and movies that use the plots of myths and Grimm's fairy tales, and frankly, I think they're quite boring and horrible. However, to a literary geek such as myself, these recurring ideas can be important. Knowing the patterns of literature can make writing <drumroll> THE GREAT AMERICAN NOVEL </drumroll> a lot easier, because if you have a powerful basis for your story, then you can focus on the details, which will make the meaning that you want to convey "pop." For a slightly more concrete example, someone who is new to programming will think their teeny "Hello World!" program is amazing because they made it themselves. It's a valid reason to celebrate, but Bill Gates will not be impressed. However, someone who knows thirteen different languages inside and out will be able to create something really spectacular, even if it is very simple, because-- to sum it all up-- they know what they're doing. An understanding of how literature gets to people (isn't that its purpose?) will certainly make one's writing improve.
One particular pattern that always gets to me is the concept of the Journey. It's used mainly (and therefore might seem over-used) in fantasy books. (One series particular that comes to mind is J.R.R. Tolkien's books.) A character gets sucked into a situation because of something beyond his control (prophecy, saving the world, rendered homeless like in Eragon...) and then tests his strength/intelligence/loyalty/and perhaps even faith and in the end almost always comes out the winner. It's probably the easiest way to shove in a moral (showing how the character solves the problem is vital), but what intrigues me personally is the element of choice. It strikes close to home when a character is shoved into a situation he doesn't really like. After all, we're all alive, aren't we? We go to school, get jobs, get sick, have our hearts broken. He has tough times, he uses his ingenuity to get through it, and in the end comes out all the better for it. This message of hope is vital: we keep on truckin’, do what we gotta do, and we’ll be rewarded. We might not always like what is going on around us, but this sense of purpose is important to us as humans. We like to solve problems (crosswords, giving advice to friends, inventing gadgets), but in the novel this is made more dramatic by giving it a sense of urgency, and life-or-death importance. We don’t often experience this type of pressure in real life (heaven forbid!) but we like suffering along with our character because it gives us faith that our life has purpose too. We might not have prophecies being made about us (although you never know), but being “important” is a basic human need. It comes out in our desire to be famous, or change the world, and since literature is a study of what it means to be human, it pops up there too.