Another major control instruction that we can have problems with is QNH. Over the winter months (particularly over the last week) the QNH has been hovering in the low 980's millibars. In the past I have had operators who are used to Inches actually read back "roger on the inches" , and the obvious problems become obvious (London TMA's transition level is very much lower than some pilots may be used to as well).
Readbacks are something we do require, being Tower, TMA or Area, because in the subsequent incident report, it all sounds so bad when standard R/T isn't used, and questions start being asked!