Mind design space refers to the configuration space of possible minds. As humans living in a human world, we can safely make all sorts of assumptions about the minds around us without even realizing it. Each human might have their own unique personal qualities, so it might naively seem that there's nothing you can say about people you don't know. But there's actually quite a lot you can say (with high or very high probability) about a random human: that they have standard emotions like happiness, sadness, and anger; standard senses like sight, vision, and hearing; that they speak a language; and no doubt any number of other subtle features that are even harder to quickly explain in words. These things are the specific results of adaptation pressures in the ancestral environment and can't be expected to be shared by a random alien or AI. That is, humans are packed into a tiny dot in the configuration space: there is vast range over of other ways a mind can be.