I will share some words of wisdom from a superstar of my field, a good man and thorough.
1. NO anecdotes and NO personal stories a la "Ever since I went to Mongolia, I can't stop dreaming of Genghis Khan and his aggressive foreign policy" or "When I was five I first tasted Pouligny-Saint-Pierre. I am yet to spend a day without thinking about bacteria". I'm being facetious, of course, but you get the point. Unless one's personal experience has direct impact on one's potential research (like field research in Mongolia, or teaching/learning a language) it is advisable not to mention it.
2. Avoid emphatic intensifiers and exercise caution with epithets. The committee is likely to be completely un-smegging-gripped if the applicant writes that hu is "very interested in this wonderful program, extremely enthusiastic about the research" or finds that "the profs are amazing and absolutely perfect for me". All these noble sentiments ought to be demonstrated or explained, "I've chosen to apply to this program for X, Y, Z reasons (extensive library resources, opportunities to do research abroad, interdepartmental collaboration, sexy lab, general strength in a specific subfield of a given department, easy access to archives etc.). The research carried out by profs A and B aligns with my own scientific interests because of X, Y and Z."
3. Cogency, brevity and coherent structure. The adcoms have plenty to read. It is best not to complicate their task by exploiting ornamental language and concocting sentences with 50 clauses. For obvious reasons, a repetition of one's CV is undesirable as well as enumeration of the courses that an applicant took throughout hus academic career. Buzz words may be deemed annoying by the weary intellectuals. The general tone of the narrative is a subjective matter. The general guideline is somewhat vague, alas: be assertive, but not smug; be respectful, but not obsequious. Make someone with good taste read the product of your writing anguish.
4. Proofread 50 million times.