Callback hell is a "loss of composition" problem. Most languages, including JavaScript, have things called expressions. Expressions rely on functions returning their output. They become useless when functions give their output through a callback passed as their input: they don't compose with that frankenstein.

Node style callbacks are a prime example of how in the quest for "simplicity" and hate for abstractions its easy to build a monster / frankenstein like caolan's async. Node callbacks were a self-contradicting philosophy: it abandoned all thats already in the language in the quest to avoid an abstraction that was not already in the language :)