I hate it when people say they don't want to tell you too much about the book or they'll ruin it... but I don't want to tell you too much about the book or I'll ruin it! I've mulled over this problem for a few days because it seems like a cop-out. So the conclusion I've drawn is that the plot is not the reason it is such a good book - the reason is really in how Morgenstern makes everything come alive for the reader, and that isn't something you can describe easily in a quick conversation. However, the plot is quite unusual so it is tempting to latch on that when trying to explain to someone why the book was so amazing.
So I won't describe the plot here, I will only say that a wonderful, whimsical, richly imaginative world was created by Morgenstern and I was enthralled by it. I enjoyed reading The Night Circus so much that I forgot to bring a critical eye to the experience; I just got wrapped up in the story which is really what it should be all about, all the time.