My Great Manager program focuses on 13 behaviors that I believe every manager needs to be successful. I am sure there are a few that people could argue aren't the most critical and there are probably a few I left off the list.
What there should be little debate about is what I call The Big 5. These 5 skills are far and away the most important ones that managers need. If your organization has a high potential employee who cannot excel in each of these 5, they should not be promoted.
The Big 5 are: Feedback, Relationships, Delegation, Coaching, and Setting Expectations.
It should come as no surprise, looking at the list, that these five things are heavily inter-related. You can't give feedback well unless you've set solid expectations. You can't delegate well without expectations and feedback. I could go on with each of them.
If a manager is struggling, focus on these 5 areas. More likely than not, something isn't right there. The other 8 skills help move the manager to another level, but these 5 should be constantly great.