They patiently, methodically and tenaciously seek the best solutions. They keep improving and know where technology is and where it is going. They hang out with great developers, share their knowledge and ask for opinions. Because they talk so much about software, they communicate and reason about programming challenges better. They have a toolbox full of methodologies, design patterns, applications, libraries, frameworks and algorithms. Great developers know there are no absolute best practices, but a set of best practices for each specific project. It's not the tools, but how and when you use them.
They care about who they work with and are interested in areas outside their specialty. Because of this, they learn to understand the client's needs and limitations, and they work well with other team members. They anticipate problems and propose better solutions. It takes 10x less time to ship a great product with a great developer than with an average one.
Great code is simple. It is the shortest path to a solution. Because it is easily read and understood by other developers, it is easy to fix and improve. Because it doesn't try to solve future problems, it is adaptable to the unpredictable future problems. It takes 10x less to add a new feature to a great code base than to an average one.