What universities want
Admissions officers are looking for students that fit their university. In general, students that can demonstrate the quality of their character and amazing potential will be highly desired.
Ideal students are future world-changers and innovators. They are bright, motivated, positive, ambitious, and compassionate. They have a sense of direction, but this does not mean they have every step of their life figured out. A desire to make an impact is great, as is a willingness to be impacted.
Most of all, be yourself. You are enough. You have what it takes. Do not write what you think they want to hear. It is okay to have "weakness." Explain how you have or how you will overcome these "weaknesses". Universities love to see how students overcome obstacles.
How to Start the Personal Statement
Other Helpful Links
Below are links to a few university admissions pages where they describe the type of students that they are looking for.