Like many of Miyazaki's movies, the heroine is a well-developed character and eventually accomplishes something very important. As much as I love the Disney movies I grew up with, the heroines often had to be saved by their love interest by the end in one way or another(even if they were the main characters). In this movie, it was the other way around.
Kiki is a very realistic character, she's often gets frustrated, fed-up, jealous, and doubtful of her own abilities. The friends she meets help her to grow in confidence and maturity.
It doesn't need flashy special effects or pointless action scenes to keep you interested throughout the entire movie.
I was around 10 or 11 when I first saw this movie.
I love how this movie has really no actual plot, yet you become completely absorbed in it. it is almost completely character driven and Miyazaki is a master at this, as well as plot driven and balanced films.
For me I like how she's trying to be accepted by other people. She's so awkward around kids her age the same way I feel when I was her age. Wanting to be accepted yet not realizing that people, like Tombo, reaching out for her is something similar to my situation when I first moved to a new country.
Oh and I really, really like the climax of the movie. The silence of that climactic scene just further magnifies the tense moment.