I have noticed this also....
Keira seems to get a stronger reaction out of people, in general, than other actresses do. A lot of the people who like her REALLY like her, and are touchy about it when someone says something negative about her. And a lot of the people who don't like her REALLY can't fucking STAND her, and will cling to any reasons they can find for feeling that way.
She's unique among other actresses in that she really is an unusual combination of characteristics. Some of them would be considered by many as "flaws", but I think that's precisely the reason why she's compelling. If she were any different, she'd be...DIFFERENT. It's like a friend of mine recently remarked to me: "Dude, I don't know what it is, but she's just kind of WEIRD. Beautiful, and weird."
Anyway, that's why Keira will get attacked more harshly at times, which in turn just annoys her fans, and causes them to fly off the handle when defending her.