A good argument in literature:
1. Narrows the paper's focus.
You could say, "Mr. Darcy in Pride and Prejudice was very prideful in the beginning but eventually learns to love." However, that's a pretty broad topic. Mr. Darcy's pride and his relationship with Elizabeth comprise the whole novel. Your topic needs to be narrowed down. For instance, you could argue that Mr. Darcy's turning point from prideful to loving occurred at a particular place in the book and provide evidence that shows Mr. Darcy's character changing in that section. You could argue that Elizabeth had a positive effect on Mr. Darcy because of her own good (or bad) personality traits and then provide evidence to show how her characteristics impacted him. The key is to be specific.
2. Goes beyond the prompt.
Often teachers provide a simple prompt as a starting point. You can use your critical thinking skills to dig deeper into the prompt and show your instructor that you have actually read and thought about the work. For instance, if the teacher gives you a prompt asking you to discuss gender roles in traditional literature, you could simply follow the prompt and describe those roles using a few quotes from the text... or, you could choose two or three specific characters to use as a case study (extended example) throughout your paper to show an in-depth understanding.
3. Focuses on the literary work, not the paper or its author.
If you begin to talk about yourself, you've lost track of the focus of your paper. Remember that you are not arguing about how you felt about something, but rather how the piece is. You are writing about the literature, not about yourself (unless your teacher specifically asked how you felt about a piece; in that case, you should answer their question).
4. Engages with specific evidence.
Make sure you can find specific examples in the text. Go beyond plot summary into ways the author said particular things. Think about how word choice matters.
5. Suggests a new way to "read" the text.
This is the all-important "so what" of a literature paper. What does your argument tell the reader about how to think about the text? A truly great literature paper does not simply point out obvious, spark-notes-level story summaries, but puts the story or the style into a new perspective.
For more information about writing a paper, visit The Writing Process.