Yesterday an American friend of mine shuttered my world by saying that these three tenses (past continuous, past simple, present perfect continuous) are interchangeable in American English.
So if I were to say, for instance, to a friend who showed up on my doorstep an hour later than we arranged:
I was waiting for you for an hour!
I waited for you for an hour!
that would be as grammatically correct as
I've been waiting for you for an hour!
Is it always acceptable to use past continuous or past simple instead of present perfect continuous? I guess it's perfectly normal in an informal conversation but would it be just as good in formal context (i.e. an official letter, a paper etc)?