While I totally agree with their choice to have Angelina Jolie as Grendel's mother, it is quite a diversion from the description in the original text. In the writings, Grendel's mother was a rather hideous mass of foetor, slime and breasts. She was absolutely abhorrent, yet Beowulf was strangely enamoured by her. That is why I can udnerstand them placing Angelina in that role. The general audience who hasn't read the text would be hard-pressed to understand why Beowulf would be tempted by a serpentine blob, but creating the mother as an obviously beautiful and alluring figure (albeit malign) is much more easily grasped.
And just for clarification, even in the film, the mother is not a straight-up human (it's only alluded to in the trailer, but it does show the mother walking and an ominous tail dragging behind her, so yeah, there's that. I do think just from first impressions that they've done an awesome job and I can't wait to see it. There are differences, but overall, looks good.