One of the eternal problems of adapting a comic book. For one, you need to square the character people know with what you want them to do: if you just go through the motions of what they always do, some fans are happy, some fans think it's dull. Challenge the character (and audience expectations) too much, and you get "this isn't my Supergirl/Superman/Batman/Nelvana Of The Northern Lights/Howard The Duck" etc.
Then, of course, you've got one 100-140min flick to pack it into, rather than a 200-page graphic novel where you can play tricks with levels of art, scripting, detail, format, and so on. And bring in enough butts on seats to pay for it.
If Gunn's clever enough to square that circle - introduce us to a beaten-up Kara who becomes the heroine the audience know and love, or an approximation thereof - that should work all round.