The problem with reviewing a show like Clannad is having to divorce yourself from the outcome you wanted to have happen and look only at the show as it was presented. When it comes to the audience having strong opinions as to how things should have concluded, from my experience this show routinely ends up firmly in the debate with different camps fervently supporting their particular "Clannad" girl and why she should have been the one to "win". Now despite the fact that this show is widely discussed and clips of all types are available on video sharing sites I will still do my best to not spoil anything for those who want to experience Clannad with fresh eyes, but do be warned I am no professional so I may inadvertently spoil something unintentionally so you have been cautioned. One final thing only from a point of personal opinion, DO NOT play the visual novel before watching the show as the show, due to necessity, has to gloss over some of the alternate arc elements in order to tell its version of the story and it does so in ways that will be disappointing if you've played the visual novel beforehand.
This is a very complex and very long story with a separate story running along side that does factor into the conclusion of the series (Clannad: Afterstory) but to avoid spoilers that is all I'll say about that. The most basic breakdown is: a disillusioned unmotivated high school senior is looking to trudge through one final year of school before entering adulthood with no real goal or hopes in mind when through a chance encounter he comes across a person that feels equally alienated from the school and her classmates. The two learn to lean on and support each other throughout various situations that occur until finally realizing that their friendship has gently swung from casual to serious without either of them really being aware of what was happening, finally climaxing in a moment that was a surprise to no one but themselves. This story is not flawless, but my only major nitpick would be the recycling of a major plot element from a previous work (Kanon 2006) when the previous telling of that "arc" was much more emotionally significant than the Clannad version.
The animation is very good and is both dynamic and consistent, I've just been spoiled by projects from P.A. Works and therefore can not bring myself to give it a perfect 10. That being said the use of lighting and effects is masterfully done to seem grounded when the story requires it to be and dreamlike when that is appropriate.
The sound is really good both in Japanese and in English but here is another place where you should be careful to no play the visual novel beforehand since you will get a mindset of how the characters sound and interact that the show does not fully capture. The best example of this disparity would be the visual novel Kotomi Ichinose is perfect and was not the same Kotomi in the show either in Japanese and definitely not in English, whereas the English Kyou Fujibayashi is comepletely made of win. Sub or dub you really can not go wrong with either for this show.
I have to be ridiculously vague in this section to both avoid plot spoilers and because the characters in Clannad are so dynamic that there is no way to full describe them without having to write a term paper on each one. I will also only cover the most significant characters as this show boasts a trainload of characters who all serve to expand and deepen an already dynamic storyline. That being said sound the spoiler alarm just in case.
Tomoya Okazaki - The previously mentioned disillusioned and unmotivated main character of the show who through a chance encounter with a classmate on the road to school one day sets in motion a chain of events that ends up reshaping his world view and redefining him as a person. He may be the best main character in terms of establishment and development I have seen thus far and easily trumps the two other male leads of the Kyoto Animation "holy trinity" (Air, Kanon, Clannad).
Nagisa Furukawa - A high school student who is effectively one grade behind due to repeated bouts of long term illness that threaten both her education and her life on occasion. She is shy and lacks self-confidence and may serve as the weakest character in the story as her submissive nature sometimes comes across as more annoying than the author intended as her meekness seems to come and go more as the plot requires than as would be fitting for her current point of emotional development.
Fuuko Ibuki - A quiet odd girl obsessed with starfish who will sometimes go into a trance-like state when her cutness/happiness circuitry overloads. Her goal is to utilize hand carved starfish to serve as defacto invitations to her older sister's wedding. She is quirky and odd and immediately endearing but more so as the eccentric friend/little sister than a believable love interest.
Youhei Sunohara - Tomoya's best friend and reality anchor dispelling sound advice when called for but mostly serving as comic relief. His misadventures become permanently etched in the viewer's minds as comedic mastery and while they do attempt to provide him with some level of depth over the course of the story at least in Clannad he stays a mostly a humorous sidekick.
The Girl and The Robot - Despite having no interaction with the characters these two characters inhabit the second story that runs alongside the main one and to avoid spoiling anything I can tell you nothing more about them except to say their lack of development is intentional and essential to the overall series. When the pieces fall into place, it will all make sense and at least the first time will floor the viewer.
Ryo Fujibayashi - The technically younger twin of the Fujibayashi sisters who is painfully shy and also an avid fortune teller though her accuracy rate is suspect at best. She is much more developed in the visual novel than the show and to really understand her inner workings I would have to direct the viewer to the visual novel as she has a very deep personality and makeup that is only hinted at during the series.
And now for the quote unquote big three...
Tomoyo Sakagami - A recent transfer student attempting to rebuild her image and also working towards a goal of saving a local grove of trees that have significance to her. She is a strong willed pseudo vigilante who uses her vast arsenal of martial arts skills to protect herself and the locals from thugs and ruffians. This is also while she is trying to both emphasize and be accepted for her femininity and also achieve a place on the student council where she feels her position will assist her goal of saving the tree grove she holds sacred. There is much much more here but it would take a short novel to cover it all.
Kotomi Ichinose - A socially awkward loner who prefers the company of books to other people and while she is a virtual genius does not even understand the basics of social interaction. She is perpetually tormented by events of the past and her arc in the series is vastly inferior to the visual novel and like Ryo Fujibayashi to truly experience who Kotomi is you have to play the VN. That being said she still is one of the most tear inducing characters when everything comes together. Also violin = death of thousands.
Kyou Fujibayashi - The older of the Fujibayashi twins though it is never established why she deems herself as such though her super tsundere personality and hyper protective nature of her friends and especially her sister Ryo allow the audience to gloss past that point. She is constantly torn between being a tomboy and wearing her heart on her sleeve and this duality makes her that much better as you watch her try to keep her distance when all she really wants to do admit things she could never allow herself to say out loud. Interestingly enough her character in the visual novel changes depending on which route you choose to follow in one being the Kyou we know from the show and in another route being a pervy lesbian which also fits her personality surprisingly well.
What else is there to say except I can not recommend this show highly enough. As I said though play the visual novel only after watching the show as the retelling of some of the alternate story arcs in the show necessarily leave many things out that will leave you disappointed slightly. If you are a fan of romance anime this is a must watch/own.
Just for the sake of stating the obvious Kyou Fujibayashi is my #1 "Clannad" girl.