I was just wondering what you guys believe makes the difference between one and the other.

Say they can both learn 'really hard' songs on guitar and play them cleanly, what else do you look for to seperate these two groups?

For me, it's probably the ability to improvise EFFECTIVELY just by looking at the notes/chords played and working out the key on the spot, as well as being able to just improvise a song in any given style in any key.