After seeing so many recommendations for it, especially (and, as you'll later see, ironically) from Katawa Shoujo related sites, I gave it a try some months ago.
First of all the art style is fucking ugly. It's so bad that I can totally understand someone not wanting to continue it purely for how fucking awful the characters look. Especially most of the females look outright derp-tarded.
But what's the real problem? The writing is complete and utter garbage. It's just terrible. Especially when the show tries to be so tragic at times; melodramatic times three. Humor is almost exclusively repetitive slap-stick comedy, in fact, the MCs best male friend appears to exist purely for the purpose of being the Butt Monkey.
The worst though are all the girl interests which have almost no interesting characteristics. They all share some oh so tragic background that requires them to be saved from our amazing MC that ends up doing every task for no other reason than out of the good of his heart. In short, he's a wonderful example for a Gary Stu who literally fixes the girls, all of which behave like mentally underdeveloped kids without real personality, while our MC, while completely bland, can at least think.
In Katawa Shoujo all the girls are actually well written characters which help Hisao with his problems as much (sometimes more) than he does help them with theirs. In fact, in KS the characters feel like actually characters with interests and what not and aren't defined purely by their disability or some tragic background.
Meanwhile in Clannad you've the most clichéd Moe Blobs. Even Nagisa who may develop the most has for the most part no interesting traits other then interest in the god damn Dango family, which of course is brought up a million times.
The weirdest thing? The last third of Afterstory is really quite good. Like, I'm not even kidding. Everything is completely shit and then the last third of Afterstory feels like it's written from a completely different studio/writer.
But I'm no good with words, so let me just link to an article that describes the issues with Clannad much better.
http://wrongeverytime.com/tag/clannad-after-story/
I definitely cannot recommend this anime. Even though the last third of Afterstory is good... that's like 1/6th of the entire show and you gotta swim through a lake of shit before you get there :/