Just to be clear about the facts behind these US/Canada/ICAO RT discrepancies:-
The US definition of RT phraseology is quite detailed. A linguist name of Steven Cushing worked on a NASA project in the early 90's to look at phraseology and how it contributed to accidents. He produced a grammar of RT phraseology, published in his book Fatal Words (U. Chicago Press, 1997).
Some years ago, we looked at and redid Cushing's grammar. We simplified it into what is called EBNF (Extended Backus-Naur Form) and implemented a parser for it in the SW tool Bison. We also corrected a number of mistakes in the grammar.
So we know the following: as a formal language, it has an unambiguous grammar (this is a technical term), we did not check whether is semantically unambiguous.
The reports are to be found at
http://www.rvs.uni-bielefeld.de -> Publications -> Research Reports, numbers RVS-Occ-01-02, RVS-Occ-01-03 and RCS-Occ-01-05.
This of course does not address the question of semantic incompatibilities of international and U.S. phraseology; neither does it address the semantic discrepancies between similar phrases, as the TSB report referenced by PJ2 does. This problem has been around for a long time. For example, there is a discrepancy between what "maintain RWY heading [after TO]" means in the U.K. and in the U.S., if I remember rightly from a Bluecoat discussion some years ago. The U.S. expects you to head xx° on take off from RWY xx, and the U.K. expects you to track xx°. Could be a problem with simultaneous operations from closely-spaced runways.
As to the suggestion of solving the phraseology problem by everyone sticking/changing to ICAO terminology, that is about as realistic (maybe less so) than trying to establish English as the universal language of business. 70% of the world will do it in any case; the other 30% will refuse for various reasons.
U.K. and Asian countries may be more familiar with ICAO phraseology because a large proportion of commercial jet flights are international operations. Whereas in the N.A., the vast proportion of flights at any airport are N.A; there is more incentive to insist the non-locals learn your legal phraseology than to tell all your controllers to throw away the handbooks and learn something different.
PBL