1. Listen for how the vocals are strategically doubled in various parts for more strength.
2. The famous Abbey Road Studios reverb is set fairly long, but is very unobtrusive, a trademark of the studio. Also listen to how it's raised at the end of certain phrases, or dropped out in certain sections for contrast.
3. There's lots of interesting things that trickle into the mix, like the headphone leakage, finger snaps, and even a few comments.
4. At 3:00 you can hear the tambourine enter on the same track, but you also hear sound effects and piano in certain places as well because they'd run out of tracks.