Dawson Casting: Normally this is to be expected with voice acting. But one of the major complaints about the English dub is that some of the voices sound WAY older than they should, especially Kyou and Tomoyo.
Fandom Nod: The CLANNAD After Story anime answered a much speculated question in the fandom: What happens if Sanae's Bread meets Akiko's Jam? "It's the ultimate combination!"
Hey, It's That Voice!: Both dubs, though the Japanese to a far greater extent. This could get a bit long.
Nagisa's voice, if tweaked a tiny bit, sounds exactly like Rena Ryuugu when she isn't shouting "USO DA!". Also quite a few traces of Mai Tokiha.
Newbie Boom: This happened with the anime. Despite the fact that both Air and Kanon were popular, this series ended up attracting more fans than the other two combined. This led to a large group of Clannad fans who were largely independent of the Key Visual Arts fandom, who hadn't even heard of the earlier works (and didn't feel obligated to check them out). This happened again with Angel Beats! a few years later.
Averted in the West, where – for the most part – those interested in Clannad were fans already familiar with the earlier releases of Air and Kanon.