To elaborate, I didn't think it was "bad", and it succeeded in its prime directive. I cried. A lot. Like, a lot a lot. I just really, really disliked the character designs. Everybody in CLANNAD looks roughly half their stated age.
I also just felt that the routes themselves were really hit or miss. Some were really sweet, lovely, and tightly written (Kotomi, Yukine), some were just OK or undersold an interesting heroine by saddling her with a mediocre story (Kyou, Tomoyo), and some were so mind-bogglingly dull and lacking in dramatic tension I wanted to rip my hair out (MEI, Koumura, the entire first half of After Story).
I'm not going to give up on reading Key's works since they are so universally acclaimed. I'm going to try Little Busters! since the character designs in that one look a little more mature.