if i was forced to say something...anything...negaitive about this album it might be that it sounds sort of dated. it sounds like a product of the 60's (which, oddly, the white album and abbey road do not).
I think this album put a real stamp upon the summer of love which was perhaps the epitome of the 60's, there was a real optimism about it that has since vanished, and upon which many look askance. I really feel that you can see a sea change compare this to Pink Floyd's monster album from 1973 Dark Side of the Moon, which was also a connecting series of songs strung together with plenty of production innovation and also has Zeitgeist, the tone of this later album is one of resignation and weariness.
The optimism, colour and imagination that is Sgt Pepper is what makes it stand out for me, the Beatles were riding a wave of the time and they and the time seemed full of hope. This is probably my favourite pop album of all time.