Make sure you tell why it would make money, why people would watch it, how the plot would play out, and what kind of actors you would need. Be prepared to answer plenty of questions, many business related.
What's different about your show that no one else has. What's your competitive advantage? You must have something that distinguishes your show from others. Whether you are pitching a reality show or a comedy show you must have something that no one else has. Know what your going to say before you pitch it!