Context is everything.
The reason Jamerson's line on "What's Going On" works so well has so much to do with everything that surrounds it. Gaye's brilliant, emotional delivery, the gorgeous arrangement, the poignancy of the lyric and the strength of the song all highlight the nuances in what the Bassist is doing.
As other contributors to this thread have already mentioned, the Bass is first and foremost a structural instrument. In a Rock context, it has a clear purpose - to bridge the gap between rhythm and melody - and it can be tricky to express an emotion when you're working within those parameters. If you're looking for some good examples of expressive playing, I'd recommend the "Hejira" album by Joni Mitchell. On this record, Jaco Pastorius manages to both support the song and play beautifully and lyrically.
You could also listen to what Eberhard Weber is doing on Jan Garbarek's "It's OK to Listen to the Grey Voice" record. It's not rock or pop, but it's a masterclass in expressive musicianship that doesn't get in the way of the lead instrument.