Everyone in the band was pretty exceptional at what they did (I'm obviously biased because I play bass, but ESPECIALLY James Jamerson. He was their only bass player for a long time and when you stop to consider the amount of "classic" songs he not only played on, but gave extremely memorable and also technically difficult basslines, it's kind of impossible to not give him the "best bass player of all-time tag.) and managed to make each song absurdly catchy and poppy, give it an absolutely fabulous groove so everyone can dance to it and also make them not that easy to play (you can't just pick up an instrument and play any of the hits from Motown. They are fucking tough.).
What makes it even more crazy is that the documentary makes it seem like on person would come in with an idea and then they just throw around ideas say "Yea, that sounds good" and just jam out a #1 single. And that is fucking ridiculous.
The singers did the same thing and I guess that's why essentially all of Motown's output from this time is so revered; everyone from the lead vocalists to the pianist to the third guitar player to the tambourine player was the best at what he did. This was especially obvious when the remaining members of the band did a version of "Ain't Too Proud To Beg" with Ben Harper.
Now, Ben Harper is definitely someone who the average person today would say has a "great voice". People definitely say "I wish I could sing like Ben Harper". As soon as he started singing it was obvious that he isn't half the singer that David Ruffin (who sang lead on the song for The Temptations) is. If you compare the two versions, Harper looks like a fucking hack. Not even that he can't hit the notes that Ruffin can, but completely ignores the inflection and timing that make Ruffin's performance so incredible.
If you play music (or if you don't and just love soul music) this is also awesome.
I guess what I'm trying to say is that everyone involved with Motown was the best and that is what makes the music so special.