Two of the 'main three' characters (Sparrow, Turner, and Swann) seemed to have experienced a personality makeover. Yet at the same time, something felt missing. It has the same elements that made the first film a hit. On paper, nothing really seems wrong with Dead Man's Chest.
It's not hard to understand why it became so popular. Fantastic characters, a great story, amazing special effects, and an all-around 'fun' feel. The first 'Pirates' film had a rare charm that most other Hollywood pictures would envy. It's always tough to create sequels to successful blockbusters and have them live up to original's name.
Dead Man's Chest is definitely not a bad film, but I'd be lying if I said I wasn't disappointed a little.