At the time of writing this, I haven't watched "After story" yet, so this review is purely based on this season.
First of all, let me say that this is one of the funnier animes I have seen. I love the main guy's ironic jokes and jargong.
"Clannad" really hooked me from the beginning with its nice graphics and stunning intro. The first few episodes were great and I thought I had found a gem. Sadly, it kind of fell apart after a while. The story lost it focus.
Thing is, Clannad focuses more on the characters' backgrounds than their present and future. It's similar to AIR in this regard, but this was not as well executerd.
Animation
Really nice looking with digitally enhanced lighting effects, one of the better animated animes I have seen. And the intro is stunning.
Too bad the character designs are not of the same high class. There is nothing wrong with them per se, but with such good graphic potential, I'd hoped for more.
Sound
Intro and outro are nice, but far from any top ten material. The voice acting is good, but nothing that really sticks out as excellent.
Story
As usual with these kinds of animes, there is a risk of a very episodic feel, with one girl falling for the main protagonist every episode. This is not the case here, luckily. There are many subplots running in parallell, and for the first half of the series, it is well directed and with a good flow.
However, after a while the story loses momentum. It might have to do with that it starts off with probably the strongest substory, which ends in a really epic way, and all the plots after that feels bland and anticlimactic.
I might complain more than I should here. I liked Clannad a lot, but it's just too freaking bad it couldn't keep the greatness up all the way.
Character
There are lots of likable characters (although there might be a bit too much focus on wierd personalities), and tons of background to dig up. Sadly, the backgrounds get a bit too much focus and get in the way of any forward development. I really hope "After story" fixes this.
EDIT: After watching After Story... yes. All is forgiven!
