Actually, playing even simple bass lines really well is pretty damn hard. In my last band, I played bass about ten percent of the time. I'm a guitar player, not a bass player, and I found that it was pretty easy to play basic bass lines (I just had to resist the urge to play bass solos when I should be keeping a groove). But, it took me a long time to actually begin to play stuff well--to be locked into a tight groove with a drummer, but still keep it interesting.
Pointing out why a good bass player is good is pretty difficult. The song either grooves or it doesn't. The same simple four note bass line can be played by two different people--one a competent bass player and one a great bass player--and the great bass players lines will lock you into to the heart of the song much more than the listening to the competent bass player. Listen to, for example, how Charlie Mingus plays the bass. Some of his lines may not be complicated (for jazz anyway), but his sense of timing and groove, his sense of how to punch a note just at just the right time, can be indescribable.
