Well, aside from the strictly musical, but still also concerning their musical career, you could also appreciate how his managing bossiness destroyed her sister's life: the physical, the mental and even the musical.
As for Karen's voice and vocality, that's "old style" singing: singing based on the distinct qualities of the voice and the its musical delivery, back when the music shaped into a product, and not the other way round. You discover and praise in Karen what you can found in dozens of other renowned, though not always so best-selling performers.
Finally, you should be well reminded that it's not The Carpenters' music that is sold at hundreds or thousands of dollars, but certain particular items being a specific support of it: the same sound, production, vowel emphasis, etc. abstracted from the original support and translated, with the exact same engineered qualities, to a different one, would have no value: it's the fetishist, affective value attached to it, the purchasing power and will to acquire it, and the market that profits from that situation, what produces such prices and, precisely, not because they have aged well, but because they have not aged at all, but not "in themselves": it's because the era that produced that "music" hasn't ended yet... it's because society, that is, the old farts with the power to retrieve their youth in a purchase, has not aged at all or, better said, is still alive and wealthy enough, and that alone is what makes that music seem "young". It's irrelevant whether the actual buyer is a younger person who admires The Carpenters': it's the generation that has kept them "fresh" for over forty years that still makes them be, literally, priced so high. As it has happened with books or other items, that could still keep going and remain "fresh" for decades, or even centuries... or die and become dust in a couple of generations.
Michael Jackson stays rather in-between, being more of the Whitney-Barbra sort to me... as for The Beach Boys... I could hate them if I took them seriously, musically, as an icon, or any way you please: they are just a joke. In Madonna, at least, one can appreciate her professional entertainer's savviness, but you can hate people who are as mindless as their tunes, and vice versa.
