I live in Tampa Florida, (United States) but I have always been interested in international travel, world history, foreign cultures and pretty generally anything related to expanding my knowledge of the world at large. To me the most interesting thing about PostCrossing is simply communicating with people I would never normally meet in places I'll probably never go.
To that end, while I do enjoy unique postcards and interesting stamps, the most interesting thing to me is learning a little about what life is like for the person sending me the postcard and what makes the place they live special. Does someone in Tokyo have an interest in astronomy? Where does the person in Paris go when they feel like going for a jog? Is there the coolest coffee shop in Sydney that only the locals know about? I've got to know!
So tell me a little about what it's like where you live! There's probably a lot of things that are different between where you live and where I do, but I'd bet there's even more that we have in common!! :]
Also as an aside, I am not picky about what type of postcard you send or whether it is in an envelope or not; as I said for me it's more about getting a feel for what the place you call home is like. And actually, if you want to send a postcard in an envelope and include some small coins from your country or other lightweight items, that would be amazing!