I would like to hear what a "busy" line sounds like to everyone here, because I think there is a loss of communication between the opinions.
I would say a player like Jamerson and the guy from The Grass Roots carry and "serve" a song better by playing as they please, rather than following the guitar like a wounded dog like most rock guys. In my opinion, here is a *wide* gap in between non-root players like Paul McCartney / John Paul Jones / Hugh McDonell and the stylings of Michael Anthony / Mike Kroeger / Lemmy.
So is "busy" determined by the amount of notes you play or the amount of different notes you play? Is "busy" when you deviate from the "lead" instrument to create interesting harmonies?
That said, I'm genuinely shocked metal guys aren't respected more around here considering how simple the typical metal bassline is. There are exceptions, but you can find a lot of: 0 0 0 3 0 0 5 [awkward pause] 0 0 0 0 0 0 0 5 5 5 5 5 etc. Most play fast, but there isn't much going on.