The structure is the most important aspect of any paper. The structure organizes events, importance, and makes the story/argument flow. What you usually see in a mediocre paper is a big lack of structure. The main points are there, but they aren't organized properly making each point lacking. A great paper sets up everything in the right order. Making you see a flow of events, and each point makes more sense.
2. Grammar
Bad grammar can be distracting to the reader. Misspelled words aren't that big of a problem between an excellent and mediocre paper. Its the words used to describe the scene and enhance your topic. Using the right words can shorten a paper in the right way. If your descriptions are too long; then your reader looses focus on what is trying to be conveyed.
3. Captivating
Captivating stories make you want to read them instead of trudging through them. An excellent paper will tell a story that makes you want turn the page over and over again. Mediocre stories convey the feeling that the author did not enjoy writing this and so will you.
I find structure to be the hardest part for me when I'm writing. Mind wanders about the topic I'm writing about, and in turn my writing dose too.