I agree with a lot of what you're saying, especially about LOTR or Harry Potter; without the huge budget involved, either adaptation would have failed to be true to the books. It would be so painful to see the Mistborn series treated like the adaptations of Eragon, Twilight, or any number of other cheesy movies out there. Add to that, the complications behind portraying Allomancy in any way, I have a hard time seeing that played out by actors without loosing some of its character or depth... I just think there's a need for these books to be illustrated instead of acted out, it would be so much easier to stay true to the story...
