Exterior is pretty nice, but yes, the interior is rather unpleasant. Try making the ceiling just one big plate rather than many different plates, as they add a strange texture which is unnatural and causes it to look bad. The floor is a good idea. Another suggestion is to make the interior objects a different color that contrasts to the walls and ceiling, rather than one that blends. This way it sticks out more and then it will look more pleasing to the eyes. Remember, sometimes simplistic is better than trying to be complex.
Overall good job.. And.. How can you live with such low framerates? That would drive me crazy only getting 10-13fps on a single build.