Darkside was brilliant on so many levels. For the band, it was their first ablum that had a theme to it. Darksides theme was the struggles of life. It appealed to many people, In "inside out" by Nick Mason, He thinks that it did so well just because it appealeld to everybody. There were so many different elements in the album that you don't really see these days anymore. The little talking bits in during the songs. They went around Abbey Road studios and asked people how the thought about the theme subjects and the album, It was a last minute thing.
I tihnk it was the fact that Roger made all the albums with certain themes, that made them so succesful