Ta-Nehisi has an interesting point about the visual failures of big-screen comic book adaptations. I think the challenge, generally, is that things that don't have to meet the test of plausibility in the form of illustration do have to look plausible when real people are acting them out. You can't get away with the anatomical distortions illustrators get away with whether they're drawing top-heavy superheroines or the Hulk, unless you want to end up with the terrible versions of the Hulk we've seen on-screen, and it's hard to pull off aliens, feats of derring-do, and general strangeness that works on the page in real-life either.
TNC mentions the Lord of the Rings movies as an epic adaptation that does work, but I don't think the book-to-film leap there is quite analogous to the comics. Most of the things that people in those books do is within our realm of understanding and possibility. Fighting is essentially hand-to-hand or with familiar weapons. The animals and other races of humanoids in the books are variations on physical forms we're familiar with. There is some magic, but most of it's translatable, or on a smaller scale. Sure, sending water in the form of horses to drown some guys on horses is magic, but it's magic that amps up a potentially natural phenomenon. Ghosts come out of a mountain to fight human men, but they still look basically like humans fighting other humans. And when we read things in a book, we translate them with our minds.
When the possibility of what humanity can accomplish, and the nature of what humanity is changes, that's where it gets hard to translate the action with human actors. And when we're given a picture of what things are that is beyond what we conceptualize people doing, and then are asked to believe it again when it's just people doing it, it's hard to make the leap. Movies were always going to have trouble making what we were willing to accept as real on the page look real on the screen.