There is nothing wrong with this movie per se, but when you watch it it's clear this isn't in the same thematic spectrum as Miyazaki's Studio Ghibli movies.
It was a cute, funny,adventurous story about a famous thief who wishes to help a princess who is about to marry an evil man. The story is solid and fun but it just doesn't even compare to the rest of Miyazaki's movies.
It is a great debut film. It is just strange to see his very first film after watching the rest of his masterpieces.