I've always had a love of postcards, so Postcrossing is a great way to experience the joy of receiving a card from the other side of the world.
I would love to receive anything that you want to send. Hand-made cards are great. I would prefer not to receive cards in envelopes. I'm also a big fan of stamps. The Europa ones are great -- use those if you can. For Postcrossers from the US, anything other than the standard airmail and first class (i.e. liberty bell) stamps would be wonderful.
My favorite themes:
-graffiti and other street art. Banksy stuff is fantastic.
-landscapes -- specifically mountains and coasts
-skyscrapers and cityscapes
-UNESCO world heritage cards.
Really, though, any card will make me happy. Please do write something about yourself on it.