I think as much as anything, it's the way she carries herself. Manson, at least in Doom, just gives off this vibe of fragility, while Ritter can sell it when JJ's punching a moving vehicle hard enough to stop it or something. Also, like I said, something about the neck to head proportions.
I really do need to get around to watching Veronica Mars at some point.
EDIT: Shark Night: Way heavier than what I was looking for. One of the big reasons I like shark flicks, and other creature features, is that there's generally no intentionality, no malice. There's just something hungry, and a bunch of poor fools who have the misfortune of being delicious. This one has a human behind it all, and malice aplenty. That's not inherently a problem, but when I'm in the mood for one, getting the other doesn't work well.