Promotions are generally good news all around. The manager making the decision gets to recognize the hard work of a talented employee, that employee gets more responsibility and prestige, and the company fills a vacancy without having to increase overhead.
As alluring as this can be, however, it's important to identify the right candidate before you make a decision: after all, if the wrong choice is made, not only do you have a weakness in your managerial team, you also have a weakness at the position that was vacated.
This is why it's critical not only to judge past performance, but to also think about how a particular person's skillset might fit into a different job. For example, you need somebody who is able to take criticism, but also to dole it out when it is required. You want an employee who is willing to shepherd others in their development, as well as one who can also make tough decisions when the time comes for it.
It's not an easy task, but it's one that can be made easier with a 360 degree survey.
One of the reasons that a 360 degree survey is a more effective method for identifying high potential employees is that it more accurately measures capability. Instead of being constrained to what people have done, or simply what they know, it is the best possible way to evaluate how they're able to leverage what they know in different roles. This is a key component of figuring how which people would be well-suited to managerial positions.