These are the traits that I look for when hiring designers. These are also the first things that I teach when training new designers.
Know What is Essential
Designers need to understand the soul of their product plus its constraints (including technology and platform limitations, budget, corporate goals, and their team's skills). Once they have an understanding of what they want to make, and what their constraints are, they can finally set about the task of providing solutions.
Communication
It is no good if only the designer knows what is essential; they must communicate that knowledge to the rest of the team. Designers must be able to communicate ideas verbally, in writing, and visually. They are in a key position to facilitate cross disciplinary communication within a team as they have one foot in the arts and the other in the sciences. They're a better programmer than most artists and a better artist than most programmers. They serve as a bridge, translating artist speak into engineering speak and vice versa.
Right Attitude
Designers must have an open mind. They need to be receptive to critique, others' ideas, and new ways of doing things. They should be eager to learn and to continually improve their skills and knowledge. They must be proactive in providing solutions.
This is the most important trait for newbies since it is the foundation for them getting good at any of the other traits.
Provide Solutions
At the end of the day, design is all about providing solutions. A good habit is to provide three solutions to every problem, stating the strengths and weaknesses of each solution. These solutions need to be presented through clear communication.