I would agree that this is one of the main responsibilities for any manager. Employees who are not happy will not work well together and will not be as productive as they might be otherwise. Happy employees who like their company and their work environment will work much harder. They will naturally be more productive because they will work together. They will easily take extra steps and go the extra mile. A manager who cannot maintain a positive work environment is not being a very good leader.
I always see this as a "top down" situation. A manager who cannot model and provide a productive and positive work environment gets the employees he or she deserves. I think that most people want to be positive and productive, wanting only the chance to be so in a workplace that nurtures these qualities.
I generally agree with Post #2. However, it is also true that managers are not solely responsible for creating this environment. The workers also have a great deal to do with the work environment. They have to be aware of how their actions affect the overall work environment. In that way, they share responsiblity.
This is one of the primary responsibilities of a manager. A positive work environment is not just important, it is essential to getting the most out of a workforce in any field. Managers are responsible for creating this environment through clear expectations, demonstrating respect for employees, and above all communication.