While yes, it would be pretty much impossible for any human to cut another human in half with a single sword stroke, Geralt isn't exactly human. As a mutant with enhanced speed and strength, combined with lifelong training with the sword, it is only logical to assume that he would be capable of doing things with a blade that would be otherwise impossible. As for the way that his fighting style is described in the books, it seems to me like his style changes with his mood. When he is calm, he uses much more light cuts to vital areas. When he is angry or under the influence of his elixirs, he tends to have more brutal attacks. At least that's the way I interpreted the scenes from the books. He doesn't screw around. He kills his enemies quickly and moves on.
But realistically, it doesn't really make a whole lot of sense to use light cuts to arteries and such. A lot of people are under the misconception that someone with a cut artery will die in seconds. This isn't true. Even with a sliced jugular vein, or carotid artery, it can take minutes to bleed out, which gives them plenty of time to continue their own attack. Wouldn't they be incapacitated by the pain? Well not really no, since they're gonna have one hell of an adrenaline rush, especially if you just sliced them. Adrenaline suppresses pain. So realistically it would make far more sense to dismember an opponent; even if it doesn't kill them straight away, if you cut off their swordarm, or a leg, their combat effectiveness drops close to 0. The only time this wouldn't work is against armored opponents, where you would have to aim for gaps between the plates. This is the only situation it would make sense to bleed them out.
While I do agree that there was a lot of dismemberment in the video, we should keep in mind that the game isn't anywhere close to being finished yet, and more likely then not, the gore system will be tweaked. I do however like the idea of the dismemberment rate increasing with certain potions, finishers, and perhaps heavy strikes. For all we know, the attacks shown in the gameplay demo may have all been heavy attacks.