Remember that your essay does something for the reader. If your reader appreciates what you've done, your essay will be more successful. If your reader wanted to be instructed but not moved, you may get a response such as, "Don't write so dogmatically." If your reader was expecting a "persuasive" essay but you chose to delight the reader, your reader will be disappointed. This is another reason why following the directions is essential.
Readers like writers who seem to be engaging, interesting, funny or serious depending on the context, perceptive, studious, thoughtful, good at communicating, etc.
For admission and scholarship essays, readers like writers who have a variety of positive character traits and who demonstrate that they fit well among the values of the institution.
To show that you are interesting, choose an interesting topic. Normally this means choosing a topic that genuinely interests you and explaining why it is so interesting. The reader will be more likely to enjoy going on a "trip" with you.
To show that you are engaging, engage the reader by working on a worthwhile issue in your essay. Choose a topic with significant implications, one where the outcome matters. "Did Romeo love Juliet after all? Let's look at the evidence!"--even if you conclude that he did totally love her, shaking up the issue for a while is likely to keep the reader engaged.
If you choose to be funny and the context is appropriate, try out your jokes on a test audience (friend, family, roommate) before submitting the essay. Whenever possible, give yourself 24 hours without thinking about the jokes, and then return to them to see if they still seem funny and appropriate.
See the sections below for further advice.