Ah, Clannad, definitely a beautiful series. One of my favorite series of all time, just amazing in so many ways.
The story is really compelling. The romance between Tomoya and Nagisa is really sweet, and the beautiful thing is that many of the story's themes are incredibly realistic. Sure, there are a few supernatural elements to propel the story, but the issues the characters face are for the most part very easy to connect to. The pain of tragedy, how people can come together and enjoy the simple realities of life, the true strength of bonds, and what "family" truly means, all of those are something very special that Clannad definitely shows its audience.
Great characters too. Tomoya is an amazing protagonist, with utterly fantastic development and so much character growth. He starts off as cold and aloof, not caring about the world he lives in, and understandably so. However, his interactions with the other characters, most of all with Nagisa and later Ushio, really change the way he views his own life and he comes to realize that caring about other people can truly bring about a happier future and it's something amazing. Nagisa is also a great character, as she starts off as awkward and alone, but begins to have more confidence in herself with her interactions with Tomoya and later makes very mature choices that show just how much she has grown as a person. My personal favorites are Nagisa and Kyou, but in all honesty the entire cast is great, the development the characters go through is fantastic and I really couldn't hate any of them.
Clannad's a fantastic example of why anime can create stories that aren't just mindless entertainment. Clannad has so many life lessons and, in my opinion, it's very much a life-changing series that can completely change the way you look at life. Clannad made me realize just how precious the little things are and it's easily one of the most profound stories I have ever seen.