Although I love Gokus personality, I can see why they would change it. A lot of the naysayers don't like Dragonball/Z and this film simply because it looks "ridiculous". Now imagine if you keep Goku's kind hearted,happy go lucky personality. While the Japanese and those of us who have knowledge of the series (No matter what country we originate from) know thats the way Goku acts, Americans with no prior knowledge of the anime/manga will just think he is one big naive baffon whos not "gritty and realstic" enough for them to take seriously as the "savior" of planet Earth. So because of the general expectation within the American pshyche of what constitutes as a hero in America, Gokus personality might have been changed to a dull teenager (ugh) who later turns into the brave and courageous defender of the Earth when danger threatens his family and friends to fit this American mold of what a "hero" is supposed to be. A carefree, naive, constantly happy protaganist wouldn't cut it for American audiences, after "The Dark Knight" all heroes must have a dark and complex past/history to be taken seriously apparently.