Conway,
Keep up the good work.
The most irksome thing for most controllers is when, after passing an instruction to an aircraft, the pilot asks the reason for the vector, amended level, speed reduction etc. especially after being told a generic reason like, due traffic, for sequencing, etc.
This was particularly irksome in the 90s when every crusty captain fourbars seemed to fly with Qantas and as soon as you did something to them they'd want a full dissertion of where the traffic is, your reasoning for the amendment to their track, etc. Those days seem behind us, although ocassionally one of those old Qantas habits appears again from a tech crew.
The other irksome thing for those in a busy environment is when you have to repeat an instruction to an aircraft, when no response is received, especially more than twice. This causes the controller to begin equipment checks to determine if his equipment is at fault.
The old Flight West Brasillias used to be the worst. I think it might have been because the tech. crew tended not to wear their headsets during cruise and the aircraft was very noisy on the flight deck.
It seems that a certain discount airline seems to be the most common offender in this reqard. Not sure the reason for this, and I also understand that the crew aren't just sitting in the cockpit waiting with bated breath for my next instruction, they also have things to do like checklists, PA announcements, etc.
These are the most irksome off the top of my head, but I'm sure to think of some others given some time.
Carpe Diem.