For example ATC might say: XYZ Descend to 5000 ft.
Ugh... I think proper phrasology would be "XYZ, descend altitude 5000ft"...
As for "leaving" and "left": First of all, that is what radar is for. Speaking from experience gained by some visits to ATC centers (highly recommended by the way, a real eye-opener), I highly doubt ATC would clear any other aircraft for that level/altitude unless he/she can see on the scope that the first a/c has actually started to leave that level/altitude, readback or not. Besides, ATC expects you to start the descent fairly soon once the instruction has been given. If that were not the case they would add something like "at your discretion" or "when ready" or somesuch.
Second of all, as far as I know it is not mandatory to say "leaving FL320 for FL160", a simple "descending FL160" will suffice. I might be wrong on this one though, so please feel free to correct me. ATC should know what level you are at before you start the descend anyway.
Third of all, "left" should be reserved for lateral instructions ("turn left heading XXX") and not be used for vertical instructions. Helps avoiding misunderstandings.