Varies by level. As a junior it’s being diligent and detail-oriented. As a mid-level, it’s being able to stay on top of deadlines and manage people and documents. As a senior, it’s being able to execute what the partner/client wants quickly and effectively.
As a partner it’s understanding what the client wants and coming up with creative solutions to get them where they want to go. That includes effective negotiation, understanding the client’s goals, their leverage, dynamics between players, etc., and figuring out how to get the best possible deal based on what they want.
People have said “market dynamics,” but even mediocre senior lawyers can figure those out. It’s no secret what’s market (there are private deal term studies, and public deals are filed publicly), and it’s not hard to figure out a decent argument for why your deal should be on the minority side of market if it suits your client. What separates an average lawyer from a good one is the ability to leverage that knowledge to get your client a better deal.