Craig has pointed out that Uzumaki is a supreme example of horror as a body genre (in Linda Williams' sense). Yes, its horrors and its appeal have everything to do with the human body: hybridized, distorted, violated, twisted and parodied, and grotesquely aestheticized. As Craig says, Ito's bodies become "fusion figures" which are carthartically nauseating.
Uzumaki is also horror based on obsession. In essence, it's about a town full of people who, individually and communally, become obsessed with a shape.