Essentially, a guest coordinator is a people's person. You have to be able to communicate to people from all levels. For instance you could be talking to a music director, film star or a technician. The ability to communicate and get your point across is very important. In a nutshell you have to be very assertive without being aggressive.
Secondly, research is an integral part of this profession. You have to have your ears to the ground. If you are getting a particular person on the show you have to know everything there is to know about that person. The research has to be so thorough that the interviewer should be able to ask questions which brings about a "who told you that," reaction from the guest.
In fact before getting Pullela Gopichand on the show I met his mom and she told me that the reason she sent her son into badminton was because he used to play a lot of cricket and break most window panes in the colony. In fact his uncle would come home everyday and ask his mom, "so how many casualties today." So she was fed up of that and sent him into badminton. Of course Pullela Gopichand didn't expect us to know that and he was pleasantly surprised when we mentioned it to him.
Discipline is also very important. There is a lot of paper work involved in this line. There are many guests who stay out of town and this requires you to send written communication to them. So you have to be good at writing as well.