I've never actually read the original book, so I figured I would for Christmas.
I think what really stood out to me the most is the feeling of time passing that the book conveys. With each Spirit you get the feeling of spending an entire Christmas day with them, which I don't think ever really sunk in with any of the film versions I've seen. (And along those lines, I was also amazed to see how faithful the Muppet Christmas Carol was to the original book!)
I could have really used an appendix to help out with all the cultural references, since so many of them are over a century old now. But that didn't really stop my enjoyment of the book overall.