I'll start.
I hate it when I ask a bunch of questions relating to a common subject and the responses answer only one of the questions, and it's usually not the most pertinent.
Such as:
Query: What is A? Where is B? Why is B? And how do I C if B is X?
Response: A is Y.
Sometimes I feel like I have to create a new thread for every single question just to get a complete response.