1) Notice how tight the vocals are with both Lennon and McCartney singing the vocal together at the same time. They have a few inconsistencies phrasing-wise, but you can tell that most of the tightness has come from long hours of singing with one another.
2) Listen to the distortion as the compressor begins to work. The more compressed, the more distorted it becomes. Given the state of studio monitoring at the time, chances are no one in the studio heard it, and if they did, they couldn't do much about it anyway if they wanted to use the compressor (which was probably a Fairchild or an Altec).
3) The vocals are dead dry, which is unusual for the time when a vocal swimming in verb was the norm.
4) Check out the George's lead guitar fill at the ends of the bridges. The second one at 2:10 has a mis-fingered note, not that it ever bothered anyone before.
Don't forget to check out my Music 3.0 blog for tips and tricks on navigating the music business.