I think this is a nice compromise between the obnoxiously faux "grunge look" that is being pushed for Fall and my anti-"faux grunge look" stance.
Keira's wearing a comfortable outfit that happens to feature a plaid shirt and Converse, but there's nothing contrived about this outfit, and it even looks more "girly" than "grunge". She still looks like herself with a pink manicure and highlighted hair. The clothes are a subtle nod to grunge without treading anywhere near the wannabe grunge territory.
This could've easily been a Keira 'Kurt Cobain' Knightley disaster if she'd added black nailpolish and a beanie. Not that I dislike black nailpolish and beanies, but you know, there's too much of a good thing.