Atonement is one of my favorite movies ever; I was quite sad that she didn't win the Oscar for it. I thought she did a phenomenal job.
I'm still raw about her not being nominated for The Lovely Bones -- it's still her best performance as far as I'm concerned. She would have been the youngest person in history with two Oscar noms to her name, but, sadly, I think the critical and commercial panning the film received upon release quickly sealed her fate in this regard.
She was nominated for a BAFTA again, though, so there's that.