It's interesting - I thought from the prerelease videos that they'd reined in their ambition to make sure they avoided the performance problems of G3. And it kind of looks like they've gone a bit too far, targetting PCs below XBox 360 spec.
I've only played the demo so far, too early for much of an opinion. It runs fine so far, about 40 fps on a GTX 260 at 19x12, and neither CPU core was being maxed out when I checked. I was impressed with the initial foliage/lightning/water/shadows impression although I do get a weird effect with lightning inside a nearby cave... Combat is obviously better than G3 at release, though the animation/physics is not quite A-list... If my steam copy would only unlock, I could play the damn thing...