I recently posed this question on Quora, knowing that there are a thousand potential answers - all of them probably valid when you spin them the right way. However everyone has a favorite which is born out of their own personal experience, probably some tragic PM circumstance that produced a "worst case" result.
So why bother with trying to identify each individual's "PM pet peeve"? Assuming each strong opinion is the result of some tragedy - this gives you a list of really important pitfalls to avoid - each of them THE most important in someone's eyes.
Here are a few responses from others. If you have a moment, please add your experience to the comment string...
LETTING PROBLEMS FESTER
COMMUNICATION AND RECORD-KEEPING
FAILING TO PLAN
LACK OF FORESIGHT
FAILING TO LEAD