I just watched this episode again after seeing it when it first aired, and I have to say that I think this episode was nothing short of brilliant and completely disagree with all the criticisms of Donna's characterisation and of the whole Abigail situation.
People say that the whole Abigail situation is completely at odds with this but it isn't. When Sam is on the balcony with Donna, he saysthat he's almost forgotten everythign that happened to him on the leaps, kind of a "reverse swiss-cheesing" making it clear that Sam's cannot hav an intact memory of his life outside of being a leaper while he's a leaper and visa versa. When Same met with Abigail, he'd swiss cheesed with her father and will in the space of a few days, feeling every single protective instinct and loving feeling that both of them had towards her. Plus she was the one who needed to be saved, so those feelings, coupled with the drive to "put right what once went wrong" meant that his feelings for Abigal are only natural. Remember that Al says at the end of the episode that he'll have no memory of either Abigail or Sammie Jo.
Ok now onto Donna herself, I have to say that she's about the strongest example of the old saying of "behind every great man there's a great woman" that I've ever seen.
This is a woman who has to choose between her husband knowing she's alive where he can stay completely faithful to her and having him unable to properly help others and having people die and have their lives ruined as a result.
The other option of course is to deliberately mislead her husband into thinking he's not married, that Donna isn't still with him, or even exists if the "swiss-cheesing" is bad enough and where he can potentially cheat on her with hundreds if not thousands of women unknowingly, so that countless lives and futures can be saved.
People criticise the grimace Donna made, but think about it- Donna knew what had to be done, but this wasthe man she loved, who she'd had to lead to believe for four years wasn't in his life, was never married to him. You can know that a decision is right, but when it harms the person you love, it doesn't change the guilt or the pain over havign had to make it.
I mean you listen to her wording and the tone of her voice. When Sam asked if he'd ever done anything to cheat on her, her response is that she never felt he'd done anything to betray their love. That doesn't mean she didn't feel like he'd cheated on her or acted unfaithfully, just that he did it unknowingly. Could you imagine what that must be like, knowing tht you have a spouse who loves you undeniably but because of their job they not only wind up forgetting you involuntarily, but are romantically involved with countless other people on a daily basis?
Then just as she has him again, he's forced to go back, to leave the very woman he loves knowing that if he doesn't, the friend who has saved him countless times, will himslf die. After losing someone for what seemed like would be an eternity only to have to lose them again, who wouldn't crack, be overwhelmed by their own emotions for a minute.
At the same time, Sam himself is torn, knowing what has to be done, but hating every minute of it, so much so that he pauses and when he does, Donna realises that to truly love Sam and truly let him be the man she fell in love with, she has to risk never seeing him again.
Then at the end of it all, she has to go back to ordering Al to keep her a secret, which can have only been even more painful at that point. Her only comfort being her knowledge of astronomy and using the distance of different stars as a way of bridging the time gap and feeling close ot the man she loves. I honestly believe that that last "I love you too Donna" was something she could hear deep down and came from the part of him which gave himself amnesia and leapt himself around for the greater good.
Even Al comments on what a remarkable woman she is. I honestly think they're one of film's greatest love stories.