First and most important is the creativity. You have to think "big" and come up with a unique essay. The topic alone should make one curious of wanting to know more about the story. Creativity isn't something that you will read somewhere and come apply it, no, its about your life experience and how you can express it in writings.
Secondly, vocabulary matters a lot. A nice story should go hand in hand good vocabulary. A plain essay makes people easily bored even if you have that content. When it comes to vocabulary, its good at times not to use the the obvious outdated vocabulary that everyone knows. Dig deep into the dictionary and find the synonym of the word.
Finally, arrange your words well, paragraphs and sentences shouldn't be more than 8 words. Also, don't get out of topic when writing, ensure you stick to the topic. This is always a problem and you ought to keep it in mind.