I see in the interesting postcrossing system the possibility to learn more about countries, people and their culture, their architecture and way of life. Postcards can mediate an impression of these subjects I think.
I'm interested to get postcards which show cities, buildings, landscapes, trains, trams, traffic and so on. - It would be great, if postcards were of usual size 10x15 cm.
Because I have a special place to collect them, and otherwise they won't fit in.