Seeing the results of their work. Working for efficient managers. A job that is not too easy. Being well-informed about what is going on. I think it's significant that huge salaries, big bonuses, long vacations and benefit packages didn't make the top ten.
Money is a relatively modest motivator.
There needs to be something beyond money or benefits if you want an emotional investment in the practice. Many employees have never worked in an environment in which they felt they were appreciated and could affect the direction of the business. Yet these are essential components you'll want to nurture in your staff relationships if you want to see them grow and contribute to their fullest capacities.