Why, how could I possibly think that the conclusion of Donna's story -- her memories ripped from her mind against her will, back to who she was before she met the Doctor, looking right through him, etc. -- was perfect? Very easily. It was all laid out there going back to "The Runaway Bride."
No one truly believed in Donna and thought she was more than the dim, selfish, self-absorbed twit that we met in TRB. Yes, Wilf loved her and believed that she could be something more, but it was more an intangible idea and it was obvious that even as Donna loved that Wilf saw her like that way, she believed that he was daft to think she was anything but who she was. She simply didn't believe it in him because she didn't believe it in herself because no one else did.
Then comes the Doctor, who takes her with him, who believes in her, who tells her how unbelivably brilliant she is on a regular basis, and she's growing into it, she's sorta believing it ... but not really. Why? Because people in her "real life" (aka Mum!) don't believe in her and she's lived her whole life believing in NOT believing in herself. Which TenII picked up on almost right away: She is loud and brassy, obnoxious and pushy because it's her armor, her defense over her own immense lack of self-esteem. The Doctor helped her believe that she could be something more, but he wasn't able to make her believe fully in herself, because her mother doesn't.
Now we get to "Turn Left." Donna, same dim, brassy, self-absorbed twit -- but still awesome, in my book -- from "The Runaway Bride." Then this strange blonde shows up, and crazy, horrible things start happening and there's a moment, small, but telling, when in the midst of all of this she tells her mother that things will get better, and her mother doesn't shut her down. There's a moment where we can almost believe that Sylvia believes that Donna will certainly try and just maybe succeed. Meanwhile, the blonde keeps popping up and propping up Donna. She tells her she's brilliant, she tells her how important she is, how amazing she is. How this amazing, wonderful man believes that Donna is amazing and wonderful and utterly brilliant. And in the end, Donna -- in a completely different set of circumstances, but amazing ones none-the-less, pushed by someone having faith in her -- becomes the amazing, brilliant Donna we know and love who is willing to sacrifice all to do the right thing. Without the Doctor, just with someone believing in her. She did it.
Then we come to "Journey's End," and as I said the set-up has all been there. Yes, the Doctor took her memories away of him and their travels and all that she did. But with one sentence he gave her the ONE thing she needs to become OUR Donna; someone propping her up, having faith in her. And this time, it will come from the ONE person who's NEVER believed in her before: Her mother.
- The Doctor: And for one moment... one shining moment... she was the most important woman in the whole wide universe.
Sylvia: She still is. She's my daughter.
The Doctor: Then maybe you should tell her that once in a while.
When she has Wilf, and so importantly, Sylvia truly, fully, verbally believing in her, the core of the brilliance that is Donna Noble will rise to the surface once again. She just needs a little faith ... and this time, she'll get it from the one person from whom she's always so desired that belief.
So ... perfect.