What do you think are the most important qualities in a graphic designer?

a very perceptive person... with the skills to visually execute.

Cactus Cowboy wrote: Patience and vivid imagination.

That actually sums it up very nicely.  In my experience, the clients who call you at the last possible moment demanding you drop everything else to meet a deadline made tight by their lack of planning, are the same ones who give you loose briefs, and then decide that your first draft was not what they had in mind despite them accepting the outline. So that's where patience comes into it — and I absolutely have someone in mind.   Unfortunately, this graphic would not work well on your business card

You'll know about the imagination because your head will be constantly full of ideas about all sorts of things, and you'll already have a bulging portfolio because you just can't stop yourself.    When you have an idea, make it real — you'll enjoy the process, and there are few better ways to learn.

Perception and the ability to listen have been mentioned. They are what save you wasting time on major revisions.

I also think that an old fashioned ability to draw is useful, because it will allow you to quickly conceptualize and outline ideas to your clients before committing serious time to the project.