I like this post at tor.com about the giant eagles. As Jeff LaSala says, giant eagles, what’s not to like?
Anyway, LaSala says:
I will concede that, of course, it would have been nice if Tolkien had added—among other things—a couple of helpful lines to make it clearer that the Eagles were simply not an option for this task and that the characters in The Lord of the Rings understood this. But maybe he didn’t need to, since any attentive reading will reveal certain truths.
Sure, plenty of arguments can be made against the Eagles’ involvement, but none can really be substantiated. One theory is simply that such a gambit probably wouldn’t succeed. The Eagles, while mighty, aren’t necessarily powerful enough to storm Mordor even in great numbers—Sauron’s power has grown strong again and it’s pretty likely he could handle them if they entered his land. He’s nothing if not studied; he knows of the Eagles. Plus, the great birds are physically vulnerable to the bows of Men (as mentioned in The Hobbit), to say nothing of the darts of Orcs or the sorcery of Sauron’s other servants. And do you think the Eagles themselves would immune to the One Ring’s evil?
I think that last point is decisive. There wouldn’t be much joy trading a Dark Lord stuck in Mordor for one with wings, probably.
LaSala has a good deal more to say about the eagles and their basic indifference to the other races of Middle-Earth. Click through if you’re interested and read the whole thing.