[usr1106]

Pronunciation is only one problem: Personally I find some British and most French natives harder to understand.

But native speakers from any continent can make me feel lost if they start to use a lot of idioms and slang expressions. My boss is an American who has lived abroad for a long time. I have usually no problems to understand what he means. In one phase I noticed him using an increasing amount of idioms I could not understand. When mentioning it to him he said he had had many telcos with US partners recently and that had probably inspired him to revive all kind of colorful idioms.