I have only slept on a futon on a frame on holiday - it was OK, but too thin to sleep on forever. I have never slept on a futon straight on the floor. I have slept on a mattress and an airbed on the floor, but I don't like to have my face so near to the ground. It doesn't seem very hygenic. In Japan I guess it's OK since everyone takes off their shoes!
I sleep in a wooden framed bed with a hard sprung mattress. I don't like soft mattresses.