The first time I read Chamber of Secrets, I knew the series was on the right track. The book is similar in size to the first one, has of course the same setting, and has a similar structure (Harry and friends work to solve a mystery and save Hogwarts), but it was a step up in complexity and maturity.

We learn more about all the characters, perhaps most importantly Voldemort himself, as we see his younger self as he was in Hogwarts. Ginny steps into a main supporting role in the story, where she'll remain the rest of the way. We get the dangerously foppish Gilderoy Lockheart. We learn that Hagrid's creatures truly can be very dangerous. And we see that while people can take Dumbledore out of the game, his greatest power is his ability to inspire those around him such that removing him is not the same as defeating him.

We get new themes, such as Harry struggling with fame (to which Lockheart provides a good contrast), and we get more sinister scares (the giant snake in the wall, whispering its desire to kill).

Also it's no wonder Ginny falls for Harry. One moment she's been taken beneath the school where the giant deadly snake lies and is having her life force drained by the journal-person she once thought was her best friend, and the next she's waking up to find the journal defeated, the basilisk impaled with a sword, and Harry Potter with a phoenix, holding out his hand. No one could withstand that much heroism.

As opposed to the first book, I felt much better about the conclusion to this one. Harry faces a real threat, and he overcomes it with his loyalty to Dumbledore, his courage, and his refusal to back down from evil.

(As opposed to the first one, where he and his friends overcome traps that aren't as fiendish as they should be and then Harry defeats the bad guy because his skin is suddenly lava -- I know it makes sense in the context of the whole series, but at the time we didn't have the context of the whole series. :p )

The release of Dobby at the end was a nice finishing touch.