that a ton of pop rock songs use now, the beatles went beyond and many of their progressions involved more chords or even went outside typical tonal music boundaries and into blues/chromatic. This thread sparked my curiosity, and after research I found it the Beatles actually were fairly standard with their contemporaries when it came to harmony, and didn't really do anything too radical when it came to chord progressions/key changes-

http://www.seechord.co.uk/song-writing/secrets-of-the-beatles/

I'd imagine that melodies relied on rhythmic variations (I read they switched meter in a few songs in the middle of the songs!!) and chromaticism that might make them difficult to sing. They do seem to use the traditional downward fifths to tonic in a lot of their songs.