The "catchy hook" theory for distinguishing rock doesn't really work - I mean you've got most metallica, megadeth, priest songs with catchy choruses and some rock bands with no choruses at all.
Isn't the main difference that in metal the instrument which provides the melody, the guitar, is also used percussively? Each beat or segment of a beat is occupied and emphasised by a note or chord of the guitar, while in rock the melodies are usually simply arranged or draped across a bar for melodic effect only. That's what makes metal "heavy"