Good expository writing should be clear and easy to read. The reader should not have to untangle long, awkward sentences, nor be distracted by short, choppy ones. Be sure you understand what you intend to say. Focus on your ideas, and do not put obstacles between them and the reader's comprehension. Your purpose is to communicate.
Originality is the hardest aspect of writing to define.
It is the spark that sets your paper apart from the others, makes it interesting to read instead of a chore. It may stem from an unusual topic, or from a fresh slant on the same topic half the class has chosen. It may lie in lively examples, intriguing facts, or succinct methods of expression.