In ye ol days... movies based on comics never really did resemble much its inked source.
There is always something lost and something added in the transition from one media to another, it usually takes some time to adjust this so that the people that already know the story dont get disappointed and the rest can understand the story and get attached to the characters
This is even harder to do when the product come from overseas, needing to adjust the story to western audiences, sometimes even having to adjust the targeted audience
Anime and video game movies seem to be still looking for this balance and at this moment, DBZ doesnt look to be one to hit the jackpot
Just give it some time, it usually takes a few movies to hit the nail... and then you got clones of those movies jumping all over the place, breeding with each other and creating clones of them self. Meaning that the experiment got all fucked up and they got to go back to formula