Make no mistake: Clannad is moe. To be sure, the body designs look like adults (or teenagers) rather than children, but the facial design, particularly of the girls, is very much the big-eyed, tiny-mouthed stereotypes that most people associate with anime. This isn't the kind of thing I normally like, unless it's being spoofed (as in Lucky Star), but to get anything out of Clannad you'll have to be able to deal with it.
While we're counting potential black marks against the show, it should be said that Clannad is also a high-school romance. If you can't stomach school kids failing to communicate their feelings, this show may not be for you. And finally, the show is based on a visual novel (by Key/Visual Arts). Visual novels, for those who don't know, are basically role-playing games; they often fall into the dating-sim or erotic territory, though thankfully Clannad doesn't go there.
To summarize the list of preconceptions: Clannad is a visual novel-based moe-style high-school romance. That mixture has lead to some of the most dreadful examples of anime ever created, and would be more than enough to make some viewers avoid the series like the plague.
But the thing is, Clannad does what it does very well. While its various story arcs do have a certain feel of questing about them—solve the riddle of Fuko, make Kotomi come out of her shell and come to terms with her past, find a way to get the drama club established for Negisa—the writing introduces the various plot lines some time before they become the main focus, making the structure of the series as organic as it can be, rather than a succession of stages which need to be completed. Perhaps more importantly, there is never really any question that Tomoya will get together with Negisa; other possible romances are entirely one-sided affairs, and only really occupy two episodes late in the series (in which Toyoma is suitably flummoxed by the attention he's receiving). So Clannad avoids cheap 'Which one will he choose?' dramatics, and instead allows its two romantic leads to grow together gradually. (Three of the other romances are given room to breathe in two of the specials which accompany Clannad and After Story, but they are presented as just that: alternate romances in alternate realities).
The other thing that keeps Clannad from slipping into mediocrity (or worse) is that it has underlying theme: family. Tomoyo says in episode 18:
"When I say family, I don't necessarily mean your real family—it could be your friends instead. All that matters is that you have something like a family to support you."
From this perspective, pretty much the entire show is about family, be it Tomoya's estranged father, Negisa's crazy parents—who steal every scene they're in—or her favourite song (hell, one of the plays shown late in the series is Oedipus). Kotomi's arc is a good example: although concerned with her parents, it really involves her friends gathering around her to offer support, ultimately through actions rather than words. It ends up being far more subtle than 'Tomoya solves the problems of possible romantic partners', and Clannad should be commended for that.
Tomoya himself is a genuinely nice guy with a playful sense of humour (as is true of the series as a whole), and that helps the show considerably, not least in providing a fairly convincing reason why all these girls might be interested in him (rather than wish fulfilment of the player/viewer). And as for the girls themselves… Ryon wins the Complete Drip award, as Negisa (arguably) manages to grow a spine as the series progresses. Tomoya and Kyon, as the boisterous tsundere-types, are much more dynamic and fun. Tellingly, Kyon has to train the socially inept Kotomi in the art of delivering comebacks; so guess who is responsible for more comebacks over the course of the show? But all of them, detached as they are from their possible roles as romantic interests for Tomoya, fare better than they might have otherwise. Moe or not, Clannad is mainly focussed on friends doing things together.
(Incidental aside: Kyon and Ryon are twins, and mirror the twins Kagami and Tsukasa from Lucky Star, even down to the colour of their hair, and the fact both Complete Drips (Ryon and Tsukasa) have short hair and the tsundere types (Kyon and Kagami) have long hair. It must be a tradition, or an old charter, or something).
That isn't to say that it doesn't delve into melodrama and sentimentality, especially at the climaxes of Fuko's and Kotomi's arcs, launching into Clannad's Sentimental Music Cue™ and Emotional Fireworks Display™. Whenever this occurred, I found myself disappointed; the show is, for the most part, too well written and structured for such heavy-handedness. These emotional 'pay-offs' seem more like cop-outs to me, as if the writers were unwilling to let the stores play out to their natural conclusions, and instead felt obliged to offer viewer an Uplifting Resolution™. In both arcs, the journey is rather better than the destination, and that may be true of the show as a whole.
Special mention should also be made of the haunting 'Girl in the dying world' sections; these are probably my favourite sections of the series. The understated images, the narration of the little robot, and the tone of loneliness they set are simply another reason why those descents into sentimentality just don't work for me.
Ultimately, Clannad is an often funny, occasionally cute, sometimes sentimental, but always leisurely high-school romance, bolstered by a surprising thematic depth. It isn't as funny as Azumanga Daioh, as clever as Haruhi, or as bizarre as Lucky Star, nor as complex (and generally different) as His and Her Circumstances. To be sure, it doesn't exactly rise above its origins, but it does largely play them down, for the most part successfully. If you're going to make a visual novel-based moe-style high-school romance, Clannad is pretty much the way to do it.
After Story is an entirely different beast, however.
Verdict: Very good: don't miss it. Really, unless those potential black marks I mentioned at the start are just too much for you, Clannad is definitely an enjoyable watch.
[Version watched: Region 1 English dub; review also posted at MyAnimeList]