One thing I'm beginning to notice about songs by the Beatles from approximately 1962 to 1965- the moptop era, more or less- is that they use the iii chord far more often than any other music writers I know.
For those of you who have a basic knowledge of music theory, you may remember that the iii chord is the least used diatonic chord in major keys. For those who don't know, the iii chord is the second chord in these examples, the first chord representing the root or key of a song: E to G#m, A to C#m, D to F#m, etc.
So perhaps this is one of the reasons why their tunes from this period sound so unusual compared to all other forms of pop music. Being rather unfamiliar with how chords other than the I, IV, V, and iv chords worked under normal circumstances (not to say that I didn't try to go beyond them...) until recently, even I never really thought of using the iii chord.