For me it's not about what I photographed recently, but what I developed (sometimes it can be six months or more between taking and developing). So usually by that time I have a lot of images to choose from. It's not necessarily what's best, but, as you say, what will work will with the postcard size. For example, for the latest round I wanted to print an infrared image I took with HIE, but to have it looking decent, it needed both bleaching and toning to make it work. As I am extremely busy during the school year, I just didn't have time to do it, so worked with other images that were pretty much straight prints. Most of my images are from my travels (or Japan), so I guess no matter what, they always have an "international" feel.
As for portraits, I like getting them. I don't shoot portraits, but it's nice to see what other people do - this was important for me today, as I had a chance to photograph maiko (apprentice geisha) in their okiya (geisha house) and I had a hard time keeping up with all the changes (poses and such). It made me realise that this is one aspect of my photography that I should make more of an effort with. Similarly, I've received cards from people of all kinds of different subjects that have been printed in different ways, and as a result, they've inspired me to try those very processes myself. If you like portraits, you should send them. You might become known for them. I like infrared, so I'm pretty sure most regulars in the exchange know that an infrared card is most likely from me before they even flip it to the back to check the details.