hathead52 hit the nail on the head in terms of the difficulty of explaining the album's influence, at least on folks like me. I, too, first heard it as a 13-year-old when my buddy forced me to sit in his den and listen to the album from start to finish. We just stared at each other in disbelief. The "British-ness" of Sgt. Pepper's, to which Rosen speaks, also forced me to look up new words in the dictionary and grab an Atlas. It was all so -- new.
Rosen and others left out two footnotes that I think also make Sgt. Pepper's compelling: First, the album cover itself, which was the danish with the coffee. We spent hours poring over that cover, trying to identify all the personalities and whether their arrangement pointed to any "hidden" themes. What fun! I think about that every time I look at a music CD today, and how today's generation of listeners is somehow missing out.
Also, according to a TV documentary I once watched, Sgt. Pepper's arrived after many thought the Beatles and their influence on the music scene were fading. McCartney was only too aware of that notion and, as he and the others were in the studio recording Sgt. Pepper's, he said: "Just you wait."