You obviously know how to answer this question better than I do. But one approach is to use this essay to highlight things that your professional experience hasn't alowed you to demonstrate. For instance, if you haven't had the chance to show leadership skills on the job, but you have been able to do this in something outside of work, then that's a good topic for this essay.
Re: word count, less is more! Don't feel that you need to use all 750 words. But be sure that you answer the the question.
Scott
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Co-Author, Your MBA Game Plan