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.
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.
So My Kids Can Watch: The reason CLANNAD was an all-ages game was so that Key's president could create something his kids could enjoy, as opposed to the adult VNs the company had been known for up to that point.
Troubled Production: The VN was originally slated for release in 2002, but Key took until 2004 polishing up the scenarios. The development suffered lots of setbacks, including one of the scenario writers getting stabbed partway through.
Yukine was supposed to be a main heroine in planning, but was demoted to a secondary character as her story did not match expectations. If you were wondering why she was on the early cover arts (which made it as the trope image of the main page), you know now.
Sanae was originally going to be overweight in her original character design, but was slimmed down to give her a more youthful appearance. The designers considered having an option to fatten her up.
The Fujibayashi sisters were going to be saved from a burning school building by "Clannadman." Jun Maeda came up with this scenario after the original writer for the route was stabbed and had to spend some time recuperating in the hospital.
Kotomi was going to be an emotionless girl.
Fuko was going to be a more serious character, before she was changed into the comic relief.