I'm going to try to organize this by character, since there's a lot going on in my head and I'm afraid if I don't try to put it in order I'm going to ramble incoherently.
Rose: I'm going to start with her, because I have the most to say about her. I've never been a Doctor/Rose = OTP person. I loved Rose in Season One, but by the end of Season Two, I found her 'Saint Rose of the Soulmate Society' status hard to handle. That being said, when she and the Doctor were running towards each other in Stolen Earth, I teared up a little. It goes to show what good actors Billie and David are. However, I really didn't like the way they handled her character here. Rose does not need a Doctor to be awesome. Rose is awesome of her own accord. I would have liked to think that, in the parallel universe, she would have made a life for herself and moved on. I will admit I've never had a true love nor lost one, but there has to be some point when you know that you can't get them back and it's time to get on with things. I just don't think it's a great message to send out that Rose is non-functional without this man. And how do they solve the problem of the 'parallel world, holes in the universe, can't be together' thing? They give her her own Doctor. If Rose was going to come back, I would have like to see her as kickass, take charge, saving the world Rose. Not whiny, needy, 'I'm the bestest companion why aren't you paying attention to me?' Rose. There's nothing wrong with Billie's acting, though. She did a great job.
Donna: So much love. I was watching the first episode of the Series with Ama last night and when she found out it was Donna for this Series, she said 'oh, but she's so annoying!' and I said 'no, she's awesome!'. Then I remembered that I had the same reaction when the Series started, too. So, great job writers and Catherine Tate for turning Donna into someone not only not annoying, but awesome too. I loved the DoctorDonna concept, I always love it when stories come together at the end and you go 'oh, oh, oh, I get it!'. It was a heartbreaking ending for her, though, and I cried. I was fine until Wilf offered to look up on her behalf and then I was bawling. I suppose there is some comfort in the fact that the Donna that traveled with the Doctor lives on in the DonnaDoctor with Rose and that her mother has seen how awesome she is and maybe she'll offer some positive encouragement for Donna in the future.
The Doctor(s): Great kudos to Tennant for this episode. His DonnaDoctor was brilliant and his Doctor Doctor was great, too. I really loved how he played to every character differently and treated them all with such affection. He was so proud of them and sad for them and such the father to the Children of Time. It's funny how this episode revolved around him, but it wasn't about him, really. He was the center of the web, but it was the rest of the web that did all the work. He raised his children well.
Jack: If you look at the last time we saw Jack with the Daleks, you can see just how much he's changed since the last time. He doesn't wait for orders, he gives them. He doesn't look to the Doctor solve everything, he comes up with solutions on his own. He was quick on his feat and took charge in the Doctor's absence. He was great with everyone, all the little interactions were wonderful. I don't often say that Barrowman is a great actor, but he held his own here.
Martha: Martha is how I would have liked Rose to be. If you look at most of the Doctor's former companions, they have all learned to stand on their own without him. Martha saved the world pretty much on her own in Series 3 and she was ready to do it again. Her figuring out that the 27 planets were key was very Doctorish thinking and her calling the Daleks up to give them a chance to surrender was very Doctory too. Good for you, dear.
Sarah Jane: She didn't get much to do this episode in comparison to some of the others, but I loved having her there anyway. Her interactions with the Doctor and Jack and Luke and Rose were wonderful and I cheered a little when she stood up to Davros. "I've learned to fight since then." Whoot! Also, a big hell yeah for K-9. And Luke too, with his boy genius skills.
Mickey and Jackie: I didn't know they were going to show up and I was very happy to see them. I've always loved Jackie and Mickey has grown on me over time. I'm glad Pete didn't come too. When Jackie and Mickey showed up I went 'who's staying with the baby?'. Again, they didn't have much to do, but they were nice to have for the reunion.
Gwen and Ianto: I loved the little scene in Stolen Earth with Gwen on the phone to Rhys. I loved Ianto giggling at the TV in the middle of a crisis. I loved that Tosh managed to save them from beyond the grave and that they gave her a mention. I loved the reference to Gwyneth. They did some good support acting.
Other Stuff: Great job to the writing team for remembering who knew who and who liked who and who should be introduced to who. There were a lot of characters to handle and everyone got their moment to shine. I LOVED the music during the Team TARDIS Piloting scene. And I loved that scene in general. I loved Dalek Caan. I want to adopt him and give him hugs. I like that he turned on them with pretty much the excuse of 'I LOOKED INTO ALL OF TIME AND SAW THAT THE DAAA-LEKS KIND OF SUCK'. I liked the montage of those who died for the Doctor (Jabe! I love you, Jabe!). Davros was wicked, the Daleks weren't as comical as I usually find them and fitting all the puzzle of the plot together was very well done. Oh, and Wilf, will you be my grandpa?
If you have reached the end of this, I salute you!