American Pilots laziness
As a controller in the Middle East, I have found the worst pilots to work with are the American pilots, mainly due to the fact they respond to route clearances with "WILCO" and then invariably make a wrong turn and when queried swear black and blue they were cleared that way. Hence the need for full readbacks, so we can pickup the errors before they cause a disaster. I undertand it can be long and laborious, but readbacks are there for a reason and it horrifys me to think in America they let pilots get away with a simple roger to a clearance.
Secondly the Americans are also the most guilty of not reading back the frequency on a change and then either coming back 2 minutes later to confirm the freq. or worse still dissapearing into the wrong freq. ether.
Finally the Yanks are the worst for making the initial call and then not listening again on there radio. It is a running joke that it always takes two calls to get contact with an american at least.
These observations are all made in an environment where almost half the traffic are Russians who's grasp of English is poor, but at least they try.