It's very difficult o give trite answers to large subjects, but the common theme is COMMUNICATION. You tell us what you want, and we'll do the same, and we'll meet somewhere in the middle.
To answer a few specific points raised so far:
1) If you are told to maintain a specific speed, e.g. 300 kts, then you maintain that and report it to the next controller. If you are told "Not more than 300 kts etc" then you are expected to comply with any published speed restriction points or levels (e.g. 250kts at CLN on th ABBOT arrival or 250kts below FL100) unless the next controller cancels those. Seems pretty simple.
2) As has been vociferously but impolitely put by Eastern Wiseguy, we have to sort out the traffic as best we can in our own airspace, comply with standing agreements and coordination, and try and anticipate what the next guy will want. Sometimes, because of the narrowness of airspace and lack of levels, we have to use speed control to get you all through the gap, only for that speed control to be unnecessary thereafter. An example of this is again the ABBOT arrival for SS/GW. We have one place, LOGAN, and one level, FL180, through which we can point you at the next sector. If 3 of you arrive together, we have to use vectors and speed control to achieve 5 miles (minimum) in trail at the same level. That might mean the front one is asked to do 300 or more, the second 290 and the third 290 or less. Having achieved that trail, it may well be that the approach controller decides to reduce you to 220kts for a straight in or to reduce holding times. If we had not got you in trail he would not be able to do that, and you would have to go into the sector high and hold at Abbot to lose height. One we have got you all sorted at the same level and in trail, it is a RELATIVELY simple job to vary that speed, and you should not find it strange.
3) The same is true of being put over on a heading, only to be sent direct by the next guy. This may be because we have needed you on a heading and have taken you away from your standard route. It is sometimes difficult to second guess where the next guy will send you (it might be just the next fix on route or it might be 5 further on), so it is easiest to transfer you on the heading and let him decide.
The nub of the problem is that we have a choice between doing everything standard (no shortcuts, no high speed descents, no change of level from that filed in your plan (EasyJet please note!!) etc), or we can try and work to help you. The first will mean that there is no need for us to communicate variations to the next controller, but you might find it a little restrictive. It is in most ATCOs nature, however, to be flexible, and we will vary what we do to suit the conditions and try to fit in with you. We do not have time to ring up and coordinate every single variation, so it is approved practice for us to get you to tell the next guy what we told you. Seems reasonable.
By the way, I have lost count of the number of times I have had traffic about to exit the sector which requests a change of level. To achieve this will mean a phone call to the next sector to get their agreement to the new level. This delays your change of level. Far easier to transmit the following:
"Make that request with XXX on YYY.YYY, and inform them that you are released for climb/descent by this sector".
9 time out of 10 this is read back by the aircraft, only for the phone to ring 2 mins later from the next sector asking if the traffic is released for climb/descent. Who's not communicating now?
Back to the original question:
Yes please do tell us if the great Boeing design means that you can't exceed 270kts because of a lack of drain holes or whatever, or if company arrival restrictions mean you are descending your 747 from the Far East at 250kts instead of 300 kts. Likewise, if you have an 0601 restriction at LL, tell us BEFORE we send you direct to LAM and you then have to hold for 20 mins. Stay on the route and lose time that way.
Finally: BRING BACK FAM FLIGHTS!!