"The Ghibli Museum is a portal to a storybook world. As the main character in a story, we ask that you experience the Museum space with your own eyes and senses, instead of through a camera's viewfinder. We ask that you make what you experienced in the Museum the special memory that you take home with you."
Fair enough. Still, there are areas within the museum that seem to be crying out for a photo op, such as the fabulous, furry Cat Bus, over which little kids (age limit: 12) scramble over in pure glee.
Maki has posted a detailed review that includes instructions for non-Japanese speakers for purchasing tickets (at the Lawson's!).