My review is based on how I experienced Clannad as a watcher WITHOUT the need to dissect it. So as a person that sat back and just took whatever came at him. So there's probably a lot of bias. :D
The theme is "family" in a loose way. For some it means issues concerning parents. For others it's siblings, or friends or even a group of friends. This theme will recur in every arc in some shape or form.
I'll start with the characters, because that's what attracts my attention. I tend to drop animes whenever I dislike characters or when they're unrealistic/illogical. Some people claim they're not realistic, however I think they somewhat are. I think their personalities in general are very human and realistic (in the serious parts so when it counts). Each person has their unique mind-set. I think one of the reasons I like the characters is because they're easy to relate to.
I will not be writing about all of them but as a quick example I will review a couple which I think I can do without spoiling.
Characters
Tomoya: the main character.
He's a person that has basically given up on life. He's had a rough youth and doesn't really care much about his surroundings. He basically wants fool around/pass time because he has no real goal. He acts useless as he feels he is. During the story his character will develop in a realistic way in my eyes.
Sunohara: the comedic relieve sidekick friendish.
He's basically there for comedic relieve. His role is to provide the story with some comedy (very useful, will come to this later). His personality and such is sacrificed in most situations for this. He does have a few moments in which he's insightful/useful but he's mostly there for the comedy.
Since his character is sacrificed I tend not to like him too much.
Fuko: a heroine.
An introvert socially awkward girl that does her best to support her sister. Although she has an arc, she too provides of a lot of comedy. She does act like a little girl as some people stated. Some people think of it as her charm others hate her for it. I didn't judge on it, as I just watched for the story and comedy since it doesn't bother me. There's a little more to her that I like, but I can't go into detail without spoiling.
Tomoyo: my favourite heroine.
She's a strong and determined girl. She has a goal and will do her best to reach that goal. Yet she's not blind to her surroundings and doesn't want to reach her goal hurting people in the progress. She has some decisions to make and I am satisfied with the ones she made.
Story/flow
As I wrote before the it's about family. It starts off with the main character Tomoya. He then decides to help Nagisa (main heroine) and the story evolves from there. Even though there's no real clear story line and where it will from the start, the story as it unfolds satisfies my. The characters and comedy keep me interested.
The first few episodes are introduction to the characters and setting for the plot. The first part of an arc usually consists out of a comedic part. Then problems/obstacles appear which need to be dealt with. I like that it has comedic parts in between because that keeps the anime from being very heavy. Also I like the comedy so I don’t mind at all.
Sound
I'm very pleased with the Japanese voice acting and I don't understand Japanese and that's all that I have to say about the voice acting.
The soundtrack however, is just amazing. First of all the soundtrack is quite large. The background music is just "perfect". I don't think there's been any time where I would have like the soundtrack to be different. Well yeah except for one song that I don't like... Ana... some weird englishish song. The OP en ED songs grew on me. Usually I skip those, but they really grew on me.
Why I watch(ed) Clannad:
I like the characters, to me they're interesting unique loveable and it's easy to feel empathy for them. The story is great as it unfolds without a clear indication of any precise direction as it goes.
I like the tragic parts which emphasise the good things that can happen in life. And I like to see happiness. The tragic parts just give a good contrast I suppose.
Why I think you should watch Clannad:
I want you to experience what I have. Watching it, felt satisfying and afterwards I wanted more because I liked it. The soundtrack left their marks on me in a good way. I think you might like it too. When I listen to the soundtrack it reminds me of the feelings I had when the tracks played.
As a further pro I think the logic and rationale are quite good in Clannad, one of the most important things I want in a good show.
Possible cons:
Not liking the artworks, fantasy elements, or characters.
I think you should be able to decide whether you will like it or not after 2 episodes.
My background on Clannad. I've watched it 4 times. And played the visual novel once. I still enjoy watching Clannad. I think Clannad is a great anime and advise you to play the visual novel if you're seeking more dept.
