You can't listen to one entrepreneur or VC for more than five minutes without them mentioning the importance of building a great team. It's obviously great advice for a number of reasons but it is also situational. How to define the characteristics of a great employee for one company might be different for another, furthermore different companies need different types of employees. You don't need to be a computer scientist from MIT or an astrophysicist from Cal Tech to be considered a great employee. I'm coming to believe that nothing is more important to look for than sheer willpower.
A willingness to get shit done, even when they don't know how to complete a task is not something that is taught in school but is equally as rare as a degree for MIT. You want someone in your office who, when faced with a challenge will break down walls to accomplish the task. It's the person who won't take no for an answer who is going to make sure your organization moves forward, even if they have to push it themselves. What's great about that employee is not only that that they will find the answers you need but will have gained an intimate understanding of the initial problem and are now more valuable as an employee.
When you ask a Navy SEAL how they made it through BUD/S they almost all respond with some variation of "I just wanted to be there more than the guy next to me." They will all admit to not being the fastest, strongest or smartest guy in their class, it was simple willpower. Hire willpower.