I'll go with RLM's view that they did not have a sympathetic protagonist. If we could have liked and identified with Anakin, maybe the prequels would have been mediocre instead of putrid.
Oct. 12th, 2011 EDIT: The movies also lacked a strong antagonist. Darth Maul is killed off at the end of TPM. Dooku is only introduced at the end of Clones - then killed off at the beginning of Sith! Grevious is introduced in the last film...and just sucked.