To be clear, I'm by no Tolkien expert, so perhaps it's less annoying to me. I know that other series like the Expanse or GoT sometimes irritated me a little because they made changes that I thought were not necessary. Now I'm just thinking that it must be normal for an artist to adapt other's work.
I don't think it's just about the changes to established lore for most people. It's the overall quality of the show. For a series that has one of the largest budgets in television history, many people expected more from it.
So about lore: I've tried to search a little about elves, skin colour, the origin of mithril and a few other topics that were criticized, and I've watched a few video like those exchanged earlier, but I didn't find anything offending. I first missed the fact about "the lost silmaril" and the origin of mithril in Khazad-dûm... I agree it wasn't necessary to add that part about the song - which may or may not be a pure invention, I don't know but I guess so. In general it doesn't shock me more than what I saw in other series inspired by books.
Personally, I don't care who they cast as long as they fit the part and can act. That said, the cast is pretty meh. I don't mind that they're not big-name actors, but the performances are average at best. Again, in comparison to the budget, it's disappointing.
As for skin color, Elves were described as "Tall, fair-skinned people". I think diversity is great, but why cast Black actors as elves when there multiple other races to cast them as? It's like they said "Ok, we have to have at least one Black actor for every race." Why not just cast more as humans, dwarves, etc?
If they had been described as "Tall, dark-skinned people" by Tolkien, and they used Caucasian actors, the controversy would be much bigger.
About epicness: Numenor and Khazad-dûm are impressive enough, it must have been quite the masterwork for those people. But Lindon looks less dramatic partly because it's eco-friendly, but also it falls short of what Jackson's movies showed of elves' greatness. The path Galadriel is following is something too, so is Adar's and what Celebrimbor set out to do. All that is only being developed but I'm expecting more to come.
That's not really what I meant by epicness. It goes beyond just the visual representation. It's also about the way the stories are told, the actions of the characters, the way they spoke, etc.
To me, it's obvious they just tried to appeal to as wide of an audience as possible. For example, hobbits are a big part of the show, but hobbits are never even mentioned in Tolkien's stories about the Second Age of Middle Earth.
However, a large part of the mainstream associates anything Tolkien related to hobbits for obvious reasons, so the showrunners felt like they had to shoehorn in a bunch of hobbit silliness to appeal to more people.
Considering the budget, maybe they didn't have a choice.
Overall, it doesn't feel anywhere near as grand as Tolkien's writings. I realize it's naive to have expected otherwise, but there are just so many things they could have done better.