Every professor evaluates papers differently. Some are looking for creativity and don't care if your paper has a couple typos. Others are sticklers for grammar and spelling, and take off points for a misplaced comma.
In almost all cases, however, there are a few common factors that you can pretty much assume any professor will want to see in a paper.
1. Did you understand the assignment? Read the assignment carefully as soon as you get it. Make sure you completely understand it. If you're unclear, ask your professor questions right away. Also, pay attention to verbs, because they tell you what to do. There's a big difference between "summarizing" and "comparing," for instance. Understanding the assignment is super basic, but can be easy to screw up.
2. Did you make an original argument and support it? Nearly all college papers need an original argument (or thesis) and evidence that supports it. Welcome to college writing life. Professors want to see that you can formulate an opinion and use research to back it up. Your mission is to convince your professor of your way of thinking.
3. Did you show that you learned something through the assignment? A writing assignment is a learning experience. Professors create writing assignments because they want you think about something in a certain way--so, use your head. Your paper should be thoughtful and informative. It should look like you spent time on it; and didn't throw it together three beers deep.
You'll be in good shape if your paper hits all these points. That being said, this list is not a foolproof strategy. Your professor might want to see other things, as well. The better you understand your professor's requirements, the less confusing (and painful) writing can be.
Word to your flocker.