Here's the central problem with Clannad: it wants to tug your heartstrings, so it doesn't think about anything else. Unfortunately for this show, this means that it ends up being utterly contrived, unrealistic, and outright stupid.
This show expects us to believe that teenage girls act like five-year-olds. It expects us to care about these infantile, one-dimensional characters. It expects us to have genuine emotional reactions to the 'blossoming relationships' between our bland protagonist and these excuses for characters.
Is this too harsh? Let's look at the individual components that MAL expects us to rate anime by.
STORY: To be fair, the anime didn't have much to work with, given that it was based on the original VN plot. Still, the outlandish (one word: Fuuko) and emotionally manipulative plot elements (most of the backstories that were explored) weakened what might otherwise have been a sweet, unpretentious slice-of-life drama/romance.
I mean: a story about a girl who is shy and has trouble making friends? Yeah, cool, you can watch Kimi no Todoke or something. A story about a girl with Mysterious Anime Disease who has the intellectual capacity of a child? Sorry, it's not working for me. [4/10]
ART: High budget, great production values and art direction. Squishy moe character designs may not be everyone's cup of tea (Sanae looks barely older than Nagisa), but again, that comes with the territory. If anything, this aspect of Clannad makes me despair, because I can think of a lot of better anime series that deserve to have this sort of budget. [8/10]
SOUND: Evocative without being overdramatic, for the most part. Unfortunately, all the female characters sound like babies. [6/10]
CHARACTER: This is probably the greatest problem I have with Clannad, and again, it's thanks to the source material. Few of the characters get much development. All of them (the female ones, anyway) exhibit some degree of fanboy-pandering. Kyou's tsundere-ness, Fuuko's quirkiness, Nagisa's unending sweetness, etc. - not to mention the fact that Fuuko, Kotomi and Nagisa, at least, sound at least half their age. Their childish and insecure natures (especially early in the series) make for unrealistic characters - and therefore compromise this show's unrelenting attempts at making us care about them.
It's also hard to understand Tomoya's motivations at the start of the series, i.e. why he even bothered with/cared about Nagisa et al. We get no insight into his motivations at all. Why would a so-called delinquent suddenly take up this project of helping a random girl he's never met before? Why does he find her interesting? What is the point? [3/10]
ENJOYMENT: Obviously higher if you're a moe fan, but those who derive little enjoyment from watching cute girls doing cute things might find this series rather trying.
Those who watch anime with their brain switched on will find this series trying.
Those who think that female characters should actually resemble, you know, real people... yeah, don't watch this show, it'll make you rage. [5/10]
If Clannad weren't so obviously manipulative, it might be a better series. Which is not to say that the series is obviously cynical - on the contrary, I think it exhibits a genuine warmth that lesser harem-type series lack. Yet the series ultimately feels contrived and heavy-handed, and its unrealistic characters undermine its would-be heart-warming nature.