First, he is surprised to find that their minds are so occupied and they are so easily distracted by their thoughts that they actually require servants called flappers to gently strike them on the eyes when they need to remember to watch where they're walking, the mouth when they need to speak, or the ear when they need to listen.
Second, Gulliver is surprised by how obsessed the Laputians are with math. They cut their food into geometrical shapes and fashion their clothes in the same way. They have no sense of imagination or whimsy, but they enjoy drawing and mapping figures and shapes (though they are terrible at transitioning the work they do with pencil and paper into real, three-dimensional shapes like houses).
Thirdly, the people are constantly alarmed by the movement of celestial bodies, believing that the sun could at any moment crash into the earth or vice versa. They are so concerned about dangers like these that they cannot sleep well or enjoy the regular amusements of life that we do.