To me a good bassline is one which locks into the groove and provides a platform for the other parts in a piece of music. Playing with good feel for rhythm, dynamics and harmony - not just playing appropriate notes but playing notes appropriately (accents, staccato/legato, hammer-ons etc.) i.e. Playing for the benefit of the whole piece and not just showing off.
Good bass players are the ones who play good basslines consistantly.
Million-notes-a-minute, crazy slapping, tapping etc are all impressive skills and take years to master - but in my opinion these skills alone do not a make a good bass player.