One lore change in ROP that does make me sad is a particular aspect of Celebrimbor's death: that he died protecting the Nine, instead of the Three.
In the show, he dies protecting the Nine. But hours later, Sauron ends up with the Nine anyway, which makes Celebrimbor's sacrifice feel...not hollow...but futile.
On the other hand, in the book, he dies protecting the Three. And Sauron *never* gets his hands on the Three, in large part because of Celebrimbor's bravery and determination. It makes Celebrimbor's sacrifice feel all the more triumphant and crucial, in that he actively thwarted Sauron from gaining even more power and influence than he already had.
It's a little thing, but it always makes my heart feel heavy and sad when Sauron picks up the bag of the Nine Rings, because in the end Celebrimbor died for something Sauron was going to get anyway.