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.
How about you descend at exactly the same point every time?
The sequence isn't always set early enough to allow this and giving you a delay comes a fair way down our list of priorities - separation, coordination and other routine tasks take priority.
Given max speed to the field after having just reduced to 250 kts at 10000.
A sequence is a dynamic thing and approach often don't look out very far, particularly when busy. They see a gap and try to fill it because it creates another gap behind you and gives them wriggle room.
It could be you're late and the guy following is early - for us the best solution is to speed you up as it doesn't reduce the gap behind the other guy.
Given a speed reduction just prior to a height requirement, with no height waiver.
The height requirement is there for separation. If you can't make it say so and expect a vector. We understand you may not be able to make it but usually can't waive the requirement. If we don't slow you you'll run up the backside of an A380.
Slowed down to min speed by one sector, only to be given max speed a few minutes later by the next.
Probably due to a sequence change - the aircraft ahead has given it away and has no hope of making the required time. It's either lose a slot or shove you in it.
Some International heavies are very good at this - they end up low and slow and already a minute late at 100 miles (which is about when you're being handed off to arrivals so is when you get given the max).
Far easier for all involved to not flog the dead horse and push you ahead. It's either that or you continue at min and vectored to lose another three minutes to keep you behind.
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.
It all comes down to the sequence being a dynamic thing. You might be flying along on rails but the system as a whole very definitely isn't.
Time for a car analogy: would you expect to be able to maintain spacing behind another car every time with a pre-programmed level of braking? Including when the car in front slams them on hard & then coasts the last 10 metres? Or when someone nips in front of the car five ahead?