Hail to the Trollhammeren
I like the way that trolls are a bit mysterious and there are not definite accounts of all their origins nor even a definitive account of what a troll looks like or can do. They're just big and scary. Thinking back to when I was a child, the idea of Trolls was quite frightening enough - to have them described in all their gory detail might have spoiled them. When my Nan told me that the single slab bridge with a big hole in the middle, that crossed the brook at the bottom of the back field had Trolls living under it, and that one of them had grabbed her leg one day, she didn't have to say any more about what the monsters looked like...
As with a lot of his horrors, I think Tolkien intended it that way. I'm even beginning to think he left the case of the Balrog wings unproven because he knew it was both more scary that way and because he knew it would give everyone something to argue about