A comment from a listener
As a non pilot but someone who has been listening to HF comms since the late 1950s and VHF since around 1963 in various parts of the world, as well as having spent many hours riding jump seats before 9/11, I've heard many different ways of delivering and reading back clearances and have been able to observe first hand the reaction of crews to non standard terminology. I've also read too many accident reports where non standard phraseology has played a part because of the confusion caused.
I spent a good few years facilitating discussions between various national ATC providers including the North Americans, regarding training and standards and have seen the way certain European providers train their own candidates, as well as those of other nations. Seeing the way training was done in the US, in the 1990s, highlighted a good number of differences in approach which came as an eye opener, even after years of listening to the usual rapid fire delivery of the graduates and watching traffic at various US airports.
From a non professional but well informed observer's point of view, standardisation and clarity should be paramount in an environment which is growing exponentially and where far greater numbers of flights are flying through a wider range of national airspaces with a variety of ATC accents, crewed far more frequently by a two man team, often almost strangers to each other and ever more frequently of differing nationalities.
I've heard some excellent and some appalling RT over the years but the worst still has to be one heard years ago in the southern US, approaching a very busy hub airport.
An airliner based at the hub was instructed to descend to 180, and turn right 20 degrees and report the heading.
If memory serves the read back was "OK, xx xxx down to south and go west.". The controller came back with two clicks of the mike button.
Fortunately such ridiculous shorthand is rare.