I think the greatest frustration in general is the apparent lack of foresight and coordination between sectors, and potentially a lack of understanding of how aircraft operate.
For example:
Trucking along for 3 hours only to be told at TOD to descend at a reduced speed. A little notice (don't need that much) allows for a much more efficient and comfortable descent.
Given max speed to the field after having just reduced to 250 kts at 10000.
Given a speed reduction just prior to a height requirement, with no height waiver.
Slowed down to min speed by one sector, only to be given max speed a few minutes later by the next.
Ok, so you say that prior notice is not always possible and it's part of my job to meet these requirements - that's absolutely true. I'm saying that if where possible this could somehow be addressed, both our workloads are going to be greatly reduced (less vectors,etc due aircraft unable to meet requirements) and (not that it's ATC's problem) we can potentially save millions of dollars in unnecessary fuel burn.