[Rias]

Originally Posted by Level E

To add to what Rias said:

Spoiler for Yes, this is a major spoiler summary:

Saikoroshi is the world where nothing happened. I repeat, NOTHING HAPPENED.
Dr. Irie did not make a clinic in the village. Since there is no clinic, Takano did not come to the village. Oishi had retired since there is no drive for him to continue the police force with bizarre cases never happening. Tomitake nor Akasaka came to the village either. Satoshi is still there living happily with Satoko. Shion and Mion are friendly towards each other. Keiichi is not present (although this is more due to the story taking place in 1982, but the lack of Hanyuu's presence will make it nearly impossible for him to come to the village). Satoko's and Rika's parents are alive. Since Satoko's parents are alive, Teppei never comes into the picture. The Hojou family were not singled out as the village accepted the dam building project.

Since Satoko never suffered, she never got close to Rika. Since there is no club, Mion, Rena, and Rika are just classmates (the reason for the creation of the club is explained in Matsuribayashi-hen). For some unknown reason, Hanyuu cannot enter this world and could possible disappear which means Rika will loose the person who she grew up with, a person who was family to her and been with her since birth and shared all the memories through all the worlds and years. Even more so, because of the dam project going on just fine, everyone will be separated in the future.

The world is ideal. It is the way things should have happened. In this world, Rika can forget everything. Everything, the past 100+ years of torment, is meaningless in this world.

Rika is given the choice: To forget everything and live in a world that's "perfect," or go back to the world where everything she worked for and finally won has meaning. She is given this choice because she took this for granted. She is given this choice because she doesn't realize what she sacrificed for that victory.

I'll stop there for now since I think what Rika had to sacrifice will make much more sense after Matsuribayashi-hen.

Actually...not quite.

Spoiler for saikoroshi-hen character settings:

Saikoroshi-hen isn't a world where nothing happened. Rather, it's a world where no one commited any "crime".

Spoiler for Up to Tsumihoroboshi-hen. Anything more is skipped:

Keiichi - Never needed to transfer to Hinamizawa. Recall the reason Keiichi need to transfer was that he couldn't cope with exam pressure and went on a rampage to shoot childrens with an airsoft gun. In this world, he actually overcame the pressure, and is doing quite well at school.

Rena - Parents never divorced, and never moved to Ibaraki. Since her parents are actually getting along just fine, none of that stuff that happened in Ibaraki occured. Furthermore, she actually calls herself by her real name.

Shion/Mion - Never switched during the ceremony. Therefore, they actually don't feel any guilt towards each other and gets along fine. Shion is in the Hinamizawa school, and Mion is in the private school.

Satoko/Satoshi - Since the dam project actually pushed thru, they weren't isolated by the other members of the village. They do face their "new father" problem, but in time actually gets along nicely with him. Thus, there were no need for their aunt/uncle to show up. They also live happily as a family.