So a fantasy book is "Adult Fantasy" when it has more sex, longer words, more violence and more themes. You're just giving it an age rating. There's no such thing as Children's Fantasy. There are books aimed at kids, but anyone can read them. The adult fantasy books you describe are not appropriate or accessible to kids, but that doesn't make them any better. That would be like saying that only children can watch U-rated movies and adults should watch 18's all the time because they are more sophisticated.
Are you sure, when you say it's "adult", you don't just mean you like it more?
I like reading children's and adult books. In many ways, I prefer children's books. And, yes, I'm just giving it an age rating. But the argument here is whether LotR is children's or adult's fantasy so I had to try to define what made children's fantasy (most of which isn't JUST for children anyway) and what makes an adult fantasy (okay, that definitely sounds like porn!)
I quite liked LOTR. I liked the trolls and the sense of impending doom. I liked the battles and the clarity with which Tolkien describes the battle between Good and Evil - just because life isn't that simple doesn't mean literature can't be. So it's idealist, why's that a problem? It's nice to have simple, dependable characters who you can trust to react consistently to new situations.
But mostly I liked the trolls and the battles.
Dude, I read romance novels. The vast majority of romance novels have pretty clear-cut good/bad guys. The heroes are quite villainous.. but romance is comparatively formulaic. You KNOW they're going to get together before you even open the book! You can even be sure that they're going to have a baby in an epilogue. And, if he's poor, he inherits a fortune somewhere in the middle of the book. Romance is THE most idealist genre. Men you fall in love with never cheat, the sex is always great etc. etc.
I like trolls and battles too, but just not the way Tolkien tells them.