I think the most important thing to consider (aside from basics like "try to avoid dumb mistakes") is that your application needs to make a compelling argument that you are a great fit for the job. It needs to review the main job requirements and demonstrate how your skills, experience, and passions align with those.
Too many "good" applications simply describe the applicant's experience or accomplishments. The applicant is leaving it to the other to make the connection with the job. The great applications make this connection for the reader and they make it a strong argument, not just listing accompishments.
Note: this is not easy because we are all naturally focused on our own accomplishments and how much we want the job. A great application results from putting forth the effort to start with the job requirements and work backwards to what you bring to the table that is most relevant. That's why a resume needs to be specific to the job.
Hope that helps - happy to discuss further.