As I have both lived in the resident halls for 5 years for as an RA for 4, I have to agree with all of Chad's listed qualities are right on target.
A few important qualities that come to my mind:
Be willing to work as a team on staff.
Be dependable.
Enjoy working and speaking with others. Open communication is key between residents, staff members, and your supervisors.
Be ready to work with different populations and area. Freshmen, upperclassmen, apartment style living, or suites style dorms... you never know where you may be placed.
Being creative in the ways you engage with your residents; it's not always about the social aspects of life on campus. Academic, health, life skill are also important areas to work in.
You are representing your university, whether you are on campus or not. Especially as you are a upperclassmen be aware when you are out and about, that to most people on campus you're "that RA"... even when you out at the bars, concerts, etc without the Res. Life tee on.
You may start out just looking at the RA position as a place to live but it is so much more work than just room and board. You have to at least enjoy it a little bit because there will be some days or even weeks that will be tough. But it's the little moments when your goofing off during a program, or when you are having a heart-to-heart with a resident that are so wonderful.
One of the most important parts of being a RA is creating a sense of community among your residents. It won't be so much a dorm to them but a home away from home if you manage. It isn't always easy but man when it works... man it's awesome.
Good luck : )