I had to read The Hunchback for my AP Senior English class, and I rarely like books I'm assigned to read, so I didn't even consider liking this one. Even though the beginning chapter is about as slow as a beginning chapter could be, Hugo soon captivates you with his introduction of the poet Gringoire, and the chaotic situation he falls into.
Hugo's strong point in the novel is definitely his rich characterization of Esmeralda, Quasimodo, Phoebus, Frollo, and Gringoire. You get to know each character and began to anticipate and expect their reactions. They become living and breathing.
The story itself is full bodied and gothic, and its theme is unrequited love and ultimate loneliness. The cathedral becomes an intricate part of each character's life, and is as important a presence as any.
NOTE: Disney butchered the story! (But what's new, eh?)
I'd recommend The Hunchback for the older crowd, mainly for the level of difficulty rather than for content. The content is a little more mature, but not due to any explicit sexual/graphic context.