Phenom,
I would not necessarily say that those coming directly out of their undergraduate studies are any more "phenomenal" than their other classmates. What might be an important differentiator, though, is that they have spent more time earlier on in their lives soul-searching and identifying what is they are interested in doing with their lives and careers, as well as discovering why a Wharton MBA is particularly valuable at point in their lives.
Having business-comparable experience would probably be a great asset, as some of our management core classes, for example, ask you to draw upon such experiences (or other professional equivalents) in fulfilling your class assignments.
As for demonstrating leadership (my spin on the "managerial and professional potential" question), there are really too many potential options to list. There isn't a single right answer to how you can demonstrate leadership potential. Often, though, I find that leaders are people who have been exhibiting leadership throughout their life and for whom finding leadership opportunities (in a variety of different forms) becomes a natural part of who they are - a habit, even, perhaps. What is most important is that you discover leadership/professional/managerial opportunities in what you would be doing, anyway, and not pursuing an opportunity just because you think it will help in an application.
Do what makes sense for you and your interests first and foremost. That will usually result in a better chance of you being able to speak with genuine passion about the experience; that passion and learning experience will be infinitely more valuable in an application essay.
Good luck,
Victor M. Lee
MBA Candidate, Class of 2011