Like in all of Austens novels, the main theme is marriage and finding the right partners for the singles in the upper class society of a small provincial town in England. I love the irony Austen lays in describing the ever polite conversations and visits. The realism of the story (this book could just as well have been a true story) is its main strength.
Even if this story is 200 years old, it's still appealing today just because of that. The search for the right partner just never gets outdated.