This is unusual because I am reading this book for the second time after close to 40 years (first read when I was 10 or 12 probably). Some notes...
*I am impressed with how much actual oceanographic knowledge went into the book (currents, formation of new land, temperature profile of water, etc.) given the age of the book. I also think I picked up some knowledge/trivia about the oceans from reading this as a kid (like the Sargasso Sea, or the (mythical) maelstrom).
*This is a pioneering science fiction work because of the technology of the Nautilus. However most of the events that happen in the book are pretty plausible. There is some speculative geography that is kind of neat.
*There is a mystery about Captain Nemo's motives that I had forgotten all about. But the other characters have very little character development at all. There are a bunch of unnamed crewman that never interact with the narrator (Professor Arronax). His assistant Conseil, is slavishly devoted to him and seems to have almost no desires of his own.
*There are a lot of encyclopedic listings of different kinds of fish and marine life they saw.
*The plot sort of goes from adventure to adventure, but it's pretty entertaining. This could easily be a big budget film nowadays (if you fleshed out the characters more).
*The 19th century viewpoint is most evident when they come upon an endangered dugong and decide to kill and eat it.