Keep in mind that the Ear for the Animal Language is not so much a language as it is a common means of communication with animals. People who have entire conversations in Animal kind of misses the point of the thing. Animals, with very few exceptions, lack the intelligence to form full thoughts and sentences with perfect grammar.
It's the limited vocabulary, which in itself differs from animal to animal. While a bird might have the word "fly high" (like... saying... "Me fly high, me fly fast!"), one could most probably not do the same if one was speaking in the language of the wolves (the closest you might come would be something like "Me run sky, me run sky fast!"). As for something like... say... "Mithril"? I doubt neither of the languages would support that word. The closet I can think of would be "hard lots shiny". And... "Castle"? It would most probably be "big nest for two leg" in the language of the birds.
I guess I don't necessarily buy the whole idea that characters could express more complex concepts because they are more intelligent. To me, Dorgonath's statement seems to suggest that the language isn't structured to express many things... something along the lines of trying to discuss philosophy using numbers and mathematical symbols. Or, perhaps, less hyperbolically, trying to talk about technology and contemporary culture in classical Latin... there just isn't the framework for it.