Im pretty good for jamming along, but I do actually struggle to sit down and figure out exactly what someone else is doing. If I play karaoke to a song Ive never heard before its a success if Im in groove and key within a few iterations. Definitely a skill.
I noticed Richard Bona usually sings what he's playing, just reflexively. This is something Im (telling myself Im) trying to do to improve this. Should also make lines more 'musical.'