I Knew It: Many speculated that Armando would be actually Gowther in disguise, and that the gigantic armor assumed to be Gowther would actually be controlled remotely by him. A lot of people guessed correctly about Armando actually being Gowther but almost no one expected Gowther to be so effeminate looking when he revealed his true identity. The identity of who really was in Gowther's armor on the other hand, apparently was something completely unexpected.
In the initial one shot of the manga, the character dynamic between Meliodas and Elizabeth was completely different from what would be presented in the manga proper. Instead of a Chivalrous Pervert as he is depicted in the manga proper, Meliodas was not one bit perverted and more of a Straight Man. In contrast, instead of a Reluctant Fanservice Girl, Elizabeth was a Shameless Fanservice Girl. Their roles were basically switched in the manga proper.
Word of God: According to Suzuki Nakaba in an interview with Hironobu Sakaguchi, creator of Final Fantasy, Nakaba tries to emulate the same amount of effort and love that was taken in the creation of various characters from games such Final Fantasy I and Chrono Trigger (which would explain the art style, considering how close it is to Akira Toriyama's) when creating his own characters. Nakaba has also stated in the same interview that he plans to end the manga with a "satisfying" ending. What he means by this is at the moment rather ambiguous as he mentions both Tragic Endings and Happy Endings, but at the very least he states that he plans to make an ending that will leave people saying "Wow, I'm glad I read this."