ruined dwarven city that delved too deep and unleashed shadow
hero jumping on the back of an epic monster and falling down deep
Lifted directly from Tolkien with little adaptation really.
If you want to start down that rabbit hole you're going to find stuff in every piece of media supposedly lifting something from a prior person's story. The idea of people "delving into the underdark and releasing some great "blackness" can be found all over". The idea of light warring against darkness was a storyline before even Tolkien, such that most older pantheons had gods of the sun and often they were opposed by an underworld god/goddess of darkness and death.
Then there was Shakespeare. And then... well, there's an entire webcomic named "Ripping Off King Arthur". That said... Salvatore was consistently like this: can write one good scene, then goes hackity-hack for half a book. Early on half of his characters were communicating in "me dumb fighter" dialect, later he produced so many pages of mind-freezingly awful drow emo teen diaries that they probably could add up to an entire book on their own. Most of the names? That killer yo-yo? Come on.
People never wonder How the world goes round -Helloween And even I make no pretense Of having more than common sense -R.W.Wood It's not good, Eric. It's a gazebo. -Ed Whitchurch
The lost dwarf city in Salvatore's novels was called Mithral Hall (Moria being the only place in Middle-earth where you could find mithril), so I think it's fair to say Tolkien was a direct influence in that case, at least.
Honestly, if you want to see a rip-off of the Lord of the Rings, look at The Sword of Shannara. It's so blatant a rip-off that I'm surprised it got published and that there wasn't a lawsuit.