He's not very musically inclined...
he's very "wordy" and writes decent stuff, it's just that his compositions are lacking musically and severely. I doubt he's writing them but still has a say in what is used. Most good rappers have an inherent knack for music, even though most are not musically trained.
Impeccable timing being the most common and a great understanding of when and how to use melody and counter melody in their vocals ie flow. NAS just isn't one of them. The comps he uses are boring and his vocalizations, are basically just narrative and not very musical. He puts me to sleep.