I agree with the baseline points that everyone has made that a good team compromises of individuals with good communication, are hard working and have a knowledge of the problem. These are some qualities that you would find in any good project team. As a project manager a common trait that I would look in all of my team members is reliability.
Why is think this is the most important is because we all inherently value reliability.
Every day we value:
1. Reliable cars that save time and money on repairs.
2. Reliable mail that gets delivered on time.
3. Reliable investments that deliver expected returns.
4. Reliable cell phone service to stay connected.
5. Reliable vendors who show up on time.
6. Reliable restaurants that serve quality food and give good service.

Having a team with this quality will ensure that any work assigned to a particular team member would give me consistent results and be completed in that allotted time period.
This can only be achieved if the leader of the team in turn shows reliability, delivers, and; takes forward his team and meets their expectations. This causes a ripple effect of inspiring the formation of a reliable team that create reliable services and products that generate reliable business results that produce reliable growth and profits.

What do you think is the most important trait a project team cumulatively needs to have?