I don't want anyone saying, "Wait a second. I thought x couldn't do y."
There is a lot of background to be laid down, too. I always like to know why everyone does everything, and part of that answer depends on shared assumptions of people in the character's culture. This raises issues of religion, status, laws and customs. Moreover, it is not enough to just develop a world. Everything must also provide a commentary.
All characters are like us, no matter what else; all places are like home, no matter what else. The mirror may have been filched from the fun house, but it's still a mirror.