(23/02/2012 05:37 AM)ZiNgA BuRgA Wrote:  A lot more than that - the environments and story structure are very similar.  Characters seem to have a fair amount of similarity too (level-headed, somewhat socially intelligent, main character, doofus sidekick male character, and weird girls with unique problems).
Probably a bunch of smaller things that are similar too, such as jokes where the main character exploits one of the girl's stupidity / slow reaction, but I can't come up with them right now.

Some of the stuff you mentioned are pretty typical eroge stereotypes which you see a lot of, such as the doofus sidekick male character you mentioned.

As for girls with unique problems.... well if all the girls were like totally normal girls living boring normal lives then there wouldn't be much of a story then.  However I do agree with you that stories like Kanon aren't the most realistic things in the world, the "Key formula" aspect shows through, how every girl has a sob story, etc.