Wharton 2013 - Calling All Applicants

I applied to Wharton for class of 2014 and was admitted. I hope that qualifies me to answer this question. The Job Duties description is just that, your job duties. Were you responsible for running monthly reports, selling a certain product line, interfacing with another department, etc? These are the types of things that should be described in this 1600 CHARACTER space. If you are having trouble finding the right words simply look up your job description on your company's HR website.
This should not be a repeat of your resume as your resume should be focused on what you ACCOMPLISHED in each job. Did you boost revenue, cut costs, develop a new process, win an award for outstanding results? All of these things should go on your resume.

So in summary, the application is the space where you describe what you actually do for a living (it will give context as to the nature of your work), and the resume is where you highlight your achievements. HTH.