It's hard it's soft. You pick the mattress that helps you get a good night's sleep. Although not fool proof, it seems pretty straight forward. But now there are more options to help you sleep through the night.
How about a vibrating mattress? It's sort of a throwback to mom rocking you to sleep. The idea is that the vibrations help you relax, leading to a quicker entry into dreamland. Some mattresses supposedly regulate your body temperature and others absorb moisture keeping you drier and therefore cooler. And don't forget smell. There is some evidence that soothing scents can help you fall asleep, so pillow manufacturers are producing pillows that offer just the right scent to help you fall asleep. Does this mean the next time you are shopping for a pillow, you'll be asked if you want fries (or at least a fry smell) with that?
I may not want to go to that extreme, but a mattress in motion sounds kind of fun. Not necessarily relaxing, but definitely fun.