So, the initial reaction's out of the way and I'm going to think about themes and more in-depth stuff now.
Last season, the very first thing they showed us was a time-loop and a crossing of the timeline. Martha meets the Doctor before he meets her. This is echoed in the season arc, with Saxon all around them before they've run into the Master, and with the Paradox machine and bringing the Toclafane in to kill their own past. We see a Saxon poster in the alley when the Doctor and Martha take off.
This season, we started out with timing. Donna and the Doctor just missing each other for quite a large portion of the episode. And then, at the end, we see Rose, just missing the Doctor (and I have no doubt that she's there looking for him -- she saw the alien stuff happening and investigated and she was looking all about (including glancing upward). They're just around the corner from each other. So close and neither of them has a clue that the other one is there, just like the Doctor and Donna in the beginning of the episode.
I still can't quite believe it. Rose, right there in front of me, plain as day. But, oh, she looked just as tired as the Doctor's been looking. She's missed him so much. It's clear from just the few moments that we had. And they brought in the "Doomsday" music. I believe that it was Murray Gold, the composer, who likened the beat of that music to Rose's heartbeat. Rose is following her heart, following it back to find the Doctor. Because she's always refused to stop at impossible and she's crossed so many other barriers to save him already. What's one more?
So, what's different about Rose? She still wears hoop earrings, I could see them from when she swung her head around. Her makeup's toned-down. She looks older. When she swallowed and looked up, it looked like she was holding back tears. But she's determined. That's the thing about Rose Tyler -- she doesn't give up. "Will I ever see you again?" she asks and he says, "You can't," but she refuses to believe him.
She's not giving up on him.
And, once again, I think of Shakespeare's Sonnet #116. She looks on tempests and is never shaken.
When RTD does a love story, he really does a love story.