For a very short period in the mid 70s, after I'd grown out of glam etc, and before punk changed everything, 10cc was my favourite band. Even for those of us who knew they could do such a wide variety of things, this was still quite a shock.
A good shock, but a shock nonetheless.
The unique sound comes from something that would be relatively simple these days - now you sample voices and make loops in Logic or ProTools. Back then, "sampling" involved recording to tape, and "looping" meant physically splicing a tape and joining the ends to make an actual loop. This was then wound around all sorts of mic stand or other apparatus to try to keep it at the right tension as it fed through the tape machines. They then had, basically "all" notes (well, a full scale's worth) playing at once, and used the faders to bring notes in and out. Want a Am chord? Push up thefaders on the tape machines playing the A/C/E loops, and leave the rest pulled back. According to the I'm Not in Love: The Story of 10cc documentary at times all faders were pushed up a bit for some sections, to get the massive sound you hear particularly at the fade point.
This song isn't "good' because of the production. What makes it appealing is the melody, harmony and lyrics. But what makes it unique is the production. There are other special elements too. People notice (rightly) the breathy "Be quite. Big boys don't cry" whisper in the Middle 8 (mimed by Eric Stewart in the video but actually performed by the studio secretary Cathy Redfern) - but equally noteworthy at that point is the bass line in the same section, played by (I think) Eric Stewart's co-writer Graham Gouldman.
Star Me Kitten
The R.E.M. song Star Me Kitten, from Automatic for the People, used a technique heavily influenced by I'm Not in Love. Mike Mills created vocal sample loops, had them all running at once, and "played" the chords via mixer faders, as per 10cc. They even took this on the road - when they went back on tour after the Monster release, Star Me Kitten was played this way live.