In 4 words, Captain Corelli's Mandolin was "completely out of tune" with the book. The novel provided sufficient character background to understand the relationships and personal challenges the characters needed to overcome; the script writer chose to ignore the details. In ignoring them (e.g. the priest's role initially as a buffoon, but later as a spiritual leader, the fiancee's inability to read until taught by a socialist rebel, the bomb experts advice to Corelli and his eventual ironic demise), the story and characters fall flat.
This was the first movie that I've ever walked out of midway through. Interesting cinematography, great actors do not MAKE a film; the story does.