I think the SD cards are physically too small to be a truly practical medium. They can get lost in the folds of your clothing or between the pages of a book. They make no sound when you accidentally knock one off your desk. They must be tweezed in and out of their slot with your fingertips. Making them a little bigger and more massive would actually make them more practical.
Yup. Continuing my previous comment, I actually like the form factor of Compact Flash cards. Much smaller than that, and they are just too hard to label.