Many of the biggest complaints about the series seems to be caused by the fact it follows the manga with little changes then cuts off where the first arc ended naturally. In other words, the ending is exactly what happened in the manga so they can pick up where they left off if they ever want to continue with another season by just continuing to follow the manga (unlike another anime adaptation I know *cough* Pandora Hearts). Since it keeps the pacing of the manga, it is by no means a complete story. We're given bits and pieces of story and information about the world and characters but the anime ends before that information starts coming together in the manga.
