Straightforward question. What does it? Is it playing flashy solos and lead bass like Entwistle and Cliff Burton? Being melodic and all over the fretboard like JPJ and McCartney? Riding the same 4 notes the whole song? is it creating a bass part that makes people "ooh" and "ahh"?, like Black Dog or 5:15 from the RAH? Is it simply just playing those bass parts to perfection (or near perfection)? Is it the ability to slap or pop or do some crazy tapping thing (Super Mario theme?) on an 11 stringer? One thing I do know is that it isn't using picks or fingers. I asked myself this because I'm getting pretty good some of those difficult Zeppelin songs, although I've never had a good line in any original, nor any flashy bass solos live, and obviously, IMO, I'm not the same guy who had no rhythm and just wanted to play loud 7 years ago. I guess that would've been a moment of self-reflection? Dear Mods, please move this if it is in the incorrect forum. I wasn't 100% sure.