Clannad After Story has one of the best storylines I've seen. There's a lot of comedy and drama and a little bit of action mixed in as you go through Tomoya's last schoolyear and a big part of his life. There are some real tearjerking moments as the show progresses and near the end, when you think that it can't get any sadder it completely turns around and end happily.
the animation and the sounds are still as perfect as they were in the first season, I really have to hand it to "key sounds label" for their music (they also did the music in kanon 2006). They have a great tune for every moment in the show, be it a sad or a funny moment.
The characters are the same as in Clannad, only a few new figures are introduced and some extra's join in for a few episodes but we do get a few great new stories about characters that we didn't get to in the first season like Misae (the boys dorm-mother) and Miyazawa Yukine (the girl from the reference room that serves great food and coffee).
The end did leave me a bit unsatisfied though :P I really would've loved to see more but every anime comes to an end (except maybe Naruto or One Piece :P)
in short: WATCH THIS YOU'LL LOVE IT!! IT'S SIMPLY THE BEST!!