Plagiarism

Originality is what you're striving for. You want people to go, "Wow, I've never considered the idea that moths are aliens sent to slowly deprive the human race of clothing so they freeze to death!" Or something of the sort.

You'll take flack when you do something that isn't original. For example, I'll string up a few striking similarities between Eragon and Star Wars. (note: story spoilers below)

A farmer (Luke Skywalker, Eragon) on the edge of an evil empire (the Empire, the Empire) led by an emperor (Emperor Palpatine, Emperor Galbatorix) lives with his uncle (Owen, Garrow). He is taught by a knight of a long-lost order (Obi-wan Kenobi, Brom) , learning how to move things with his mind (the Force, magic). He must save the princess (Leia, Arya). His trainer knight is slain in a battle with (Darth Vader, the Ra'zac), so he must join the rebels (the Rebel Alliance, the Varden) against the Empire to avenge him.

In the second (book, movie), he meets up with a wise old guru (Yoda, Oromis), learning more of (the Force, magic). In the end, in a showdown with (Darth Vader, Murtagh) it is revealed that he is his (father, half-brother).

So yeah, stealing other writer's work is a bad thing. However, there's a loophole.

If you steal from other authors, you're a dirty idea-stealing weasel. But if you steal from real life events, you're praised!

Now, don't be too blatant with the well-known stuff. No Trojan Horses, or anything. But there's a lot of history to grab on to. The Mongolian Horde, the Trojan War, the Roman Empire. Incorporate some things from them. You'll be a history scholar, and a good writer too!