1.) We're very forward about stuff. You sit down with someone on an airplane, and after 15 minutes, you are hearing how they had two divorces, how they've got two kids in college or (my favorite) how they gave their life to Christ and how you can avoid hellfire and damnation just like they did.
2.) We're douchebags. Especially here on the East Coast. Foreigners reading this need to know that unless they have been thoroughly Americanized, they are often too polite, too understated, and too humble about their successes.
3.) We're derived from a Greek culture, which emphasizes Arete ("Excellence in Everything"). We like this mix of being a scholar, athlete, sophisticate, and leader. This very true for undergrad, less true for a graduate research program, and somewhere in between for an MFE.
That said, your personal statement is a marketing document. You are trying to sell them on why they should let you on campus rather than the Vanderbilt Engineer with the 3.7 GPA who led the Finance club, and volunteered as a docent at Detroit Museum of Art during his summers. And while a Tsinghua, IIT, or Technische Universitat Munchen would discount much of that as irrelevant, a school like Cornell is going to take a look at that cross-product and reject the 3.7 GPA Tsinghua engineer who ONLY does engineering.
Your personal statement needs to convey that you communicate well enough in English to do well and advance in a trading floor setting, which is going to be very communication-intensive. Essays that don't use proper English idioms are a big problem.
Bottom line is that schools want to see that you are both smart and academically qualified and a fun person to have on campus. The pure engineer who spends his entire life on academics- unless he is a genius- is going to get rejected in favor of the candidate who is very smart and has a large cross product.