David Denby writing in The New Yorker puts his finger on another problem:

The boy’s voice, as (author Jonathan Safran) Foer creates it, is a babbling brook of hopes and questions and bits of information on every imaginable subject. In the novel, we can enjoy all of this heroic spieling and exploring as a form of antic play. It never occurs to us that an actual little boy, however bright, however maddened by grief, could talk this way. Oskar’s voice is a writer’s virtuoso construction, and Foer combines it with the voice of Oskar’s grandfather, photographs of falling bodies, odd dialogues, lists of numbers, garbled paragraphs, nearly blank pages, and many other typographical adventures. The novel is a kind of postmodernist collage stained with tear… Much of what Oskar says in the book is amusingly beside the point. Onscreen, however, the sound of a hyper-articulate boy talking semi-nonsense becomes very hard to take.

I agree.  The “voices” in the book did not translate well into film, which is way too literal to capture the author’s delicacy, humor, and fantastical imagination.