Motown the Musical crams over forty songs into around 2 hours and 20 minutes of performance. Divide that evenly and you have less than 3.5 minutes for each song. So where is there space for a plot? Well, there isn't really. Even the most important points in the story feel rushed, squeezed in between songs. In musicals like Les Misérables (not that I am really comparing the two), the songs entirely tell the story of the musical. Whilst the songs in Motown are placed so that they do fit in with the story, it's really not the same thing. All in all, you're left with a plot that consists of a few lines of dialogue and not much else.
It's not necessarily a bad thing that Motown is more concert than musical. But it is advertised as a musical that tells the story of Motown, with some of the music added in – perhaps it should be advertised more as the music of Motown, with the record company's story set as the backdrop. Other 'jukebox musicals' like Jersey Boys and Sunny Afternoon are balanced more evenly, featuring (respectively) just under and just over 25 songs. Audience members may expect Motown the Musical to have a similar balance, but it does not.
That is not to say that Motown the Musical is simply a few people stood on stage singing well-known songs. There is great staging, great lighting, and the energy that pulsates into the auditorium is fantastic. And actors in the production make the roles their own as much as is possible when impersonating very real and well-loved people.
All in all, if you love the music of Motown and want to see it in a sort of concert with pieces of story and context dotted throughout, then Motown the Musical is the perfect show for you. But it does lean heavily towards being a concert, and less towards a full-blown musical production.