I enjoyed Clannad for a number of reasons.
Firstly, and most importantly, the girls are awesome. Tomoyo, Kyou, and Nagisa are hot; while Kotomi and Fuko are cute. Ryou is ok IMO.
Secondly, I enjoyed the various emotions Clannad made me go through. I BAWWWWWWW'd over Fuko and parts of Kotomi's arc. I felt like a high school kid going through crushes again during the Tomoyo and Kyou/Ryou arc. I'd kill for Tomoyo to wake me up each day.
Sure, it's yet another slice of life high school anime, but to me it felt different. Were it not for the awesome characters, I seriously doubt I'd have enjoyed it as much as I did.
I admit that I also enjoyed it because it was high school, a better time. I could forget about how shitty adult life is. Because of my Clannad addiction, I've kinda neglected some stuff that needs seeing to, but I don't really give a shit.