Is it the quality of the picture or composition?
Is it the music?
The plot and story?
Characters?
An X-Factor?
Before we go any further, I would strongly encourage you to watch both Clannad and After Story, because I would have to say after watching hundreds if not thousands of episodes of TV, movies and anime, this is one of the greatest series I have ever had the pleasure of watching.
So what defines greatness in entertainment?
We watch entertainment for many reasons. To learn, to escape, to dream, to enjoy and above all to FEEL. Being able to feel the connection to the characters is the most important aspect of any medium, whether it is a book, movie or anime. There is a formula people try to use to achieve this in screenwriting or novel writing but to achieve it on the scale that is done with Clannad AS is truly rare. An anime that can move you to the extent Clannad AS has moved so many people and myself is a mark that this is one of the greatest anime or any form of entertainment ever.
The animation and music are the tools with which to tell the story and on this front, this anime is beautiful to look at and listen to. Artwork is beautiful and bright with plenty of variation to match the mood of the emotion and heighten the drama.
The music is outstanding and there is no great movie/anime without great music. The songs used in AS will stick in your mind for weeks to months.
The array of characters presented were brilliantly fleshed out and served their roles superbly. Tomoya fits the classic protagonist role of the laidback student/slacker who through love, support and tragedy grows from a child into a man, husband and father. This journey and development is the core of all human experience. The relatability of his character is why so many tears are shed (including my own) and hearts moved to share in his life. He becomes an extension of us.
Nagisa has been panned in some quarters as colourless or bland. Yet I believe she is the classic support lead. Her quiet strength and love is what stabilises the relationship. Most dramas fail in that they try to hard to be colourful and crazy in choosing the protagonists, making them unrelatable. It is the simplicity of her character that actually gives the emotional heart to this anime.
I never thought I'd like 'slice of life' and romantic animes but AS has changed my mind. It has moved me like no other movie, tv series, book or anime has ever done before. Episode 18, in my mind is the greatest episode any medium I have ever had the pleasure of watching. When you are done watching that one, come back and let me know!
In summary, AS is the most moving anime or any form of media I have ever read/seen. It has that X factor, where all the elements come together to make it into a masterpiece. It is not just the animation, sound, plot or character but rather the sum of all of it that far exceeeds what it should be able to achieve. Watching the series is a once in a lifetime experience and I cannot recommend enough that you should sit down and enjoy Clannad After Story.
