The difference between a bad manager and a good one is usually quite obvious. The manager's job is to lead and supervise people, if these people feel that there is a lack of direction and that they are unable to trust their team members, then there is a problem with the management.
However, the differences between mediocre/good managers and great ones are often much more subtle. Great managers are also great leaders and have an understanding of people, they possess the highly mysterious and difficult to define "people skills". Simply put they are able to get the people they are managing to do what they want, and do it well, without coming off as bossy or rude.