"Dzien Dobry"'s are very rare here unless, like you said with friends holding doors for the group, it's with a sort of sense of humour, a joke about the politeness of olden days.
And then we have got a lot of post here about how rude are Polish salesladies. Someone enters some small shop and is greeted by the cold stare. He, as customer, expects something like "Hello, how can I help you?". The saleslady expects "Dzień dobry". Because in Poland you are expected to say it first when entering somewhere. Even if you are the Queen - you are still expected to say it. Try it next time - you'll see.
enkidu:
I am not saying, that my opinions are universal.
Thus the question.
But nonetheless I am Polish. And I think that even if my point of view is not universal, it's better than none. :-)
Does Poland really have the 'o-kari' system of indebtedness like in Japan? Do Poles really feel that they must return the favour?
Well - it's a matter of politeness and honour. Of course - a lot of people don't care for it.
It's not an exchange. It's not like "You give me something, I give you something. Get lost."
Let's say that you help someone. This automatically makes you his friend.
If you ask this guy for a favour in the future - he just can't decline your request.
That's why my advice is: Do not make a favour to the Pole unless he asked you for it.
Of course - it's not so simple. There are a lot of exceptions and special situations, but as a "rule of thumb" this advice is sound.