the vocal qualities are brilliant. Plant had a great voice, and still has, even though he cant quite reach those high notes now.
since ive been loving you is my favourite zep track too, along with the rain song.
zeppelin covered A LOT of blues songs, for example, travelling riverside blues, and had a lot of folk stuff, Zep III being full of this.
Page is very much into the old blues singers and guitarists along with folk music which plant is also fond of. plant also takes heavy influences from world music, eg. kashmir, and his recent Mighty Rearranger album - which is a brilliant album BTW.