First of all, you’ve gotta have drive—you need to absolutely love design. Aside from that, the most important trait is to be able to embrace constructive criticism without disregarding your point of view. You need to have a keen ability to see things from the point of view of your colleagues, your client, and most importantly your client’s audience all at the same time. It doesn’t hurt to have a good amount of passion, pride, and a love for solving challenging problems.
When it comes to skills, we think that skills can be developed if you have the traits mentioned above. But if we have to list a few, you should probably be good with design software (especially Illustrator and InDesign), you should be able to quickly brainstorm tons of words/concepts at the drop of a hat (they don’t have to be good), and you should be able to understand and describe what is being communicated when you see a graphic design (in more detail than stating “It looks modern”).