I'm coming to believe that how you feel about the Doctor wiping Donna's memories can be compared with how you feel about switching off a coma victim's life support.
(Except that the coma victim doesn't normally beg you not to do it - unless they're Sam Tyler, when they spend two whole series being emo about it [deliberate Life on Mars reference *wink*]. It's the begging that makes this 100 times worse. If only she'd suggested it and forced HIM to do it, or even just had it presented as more of a choice and agreed to it.)
It's about quality of life. It's about whether you think that life at any price is better than whatever death is. Is it always better to live? I'm not sure if this question is made easier or harder depending on whether you hold religious convictions. Is choosing to die on your own terms when there is an option available (albeit a terrible option) committing suicide? If a person allows you to make the choice to die, when there is something they could do, are they a murderer? Is it right to force a person to live just because you can? It's a very personal question and how you feel about it is going to be fashioned by so many cultural and personal issues.
How you feel about Donna's fate specifically also depends on what you took the whole "she must never be allowed to know anything about it all" to mean. People have said that it wont stop her turning out to be brilliant, but what if that's exactly what it does mean? What if the only way for her not to remember is for her to live a really really boring life; stuck at home, never really doing anything ever again? What if she can never be allowed to be brilliant or important or anything more than she is at the moment he's taken her memories back to BECAUSE being brilliant or becoming more than that itself is what could make her remember? What if being brilliant, in any way, is directly linked to her dying?
She's living with a curse hanging over her head, and she doesn't even know it. She'll have to be watched and controlled for fear that she might actually be brilliant enough to work it out and remember. She will be totally reliant on those she loves lying to her and holding her back from lots of fun things that we - and they - know that she would love doing. Deliberately held back and controlled, not just for a short while, but for the rest of her life.
The Doctor can now go off and angst about this in an utterly selfish way but with a certain confidence that he's done what he believes to be the right thing. The angst is tempered by him being able to keep clinging to the self righteous belief that since she's alive that makes it better somehow. He may have destroyed this woman utterly but at least she didn't actually die! Phew! Davros isn't quite right about him at all! He is still sane(!)
Donna's family are the ones who would need to actually live with his decision and do the dirty work of looking after her. The Doctor doesn't have to look her in the eye everyday and lie just by not saying anything. They could be good at it. They could work out how to allow her enough freedom to be just a bit brilliant, but what if they were to become over protective? Out of love and fear, they've been given a license to be the ones holding her back even further than they might need to. Her mum telling her that she's great is pointless if she's too scared to allow her daughter to be great. They would become empty words, and that's more useless than never saying them at all.
Donna is trapped now in an alternate reality - as much as Rose is - and a virtual world - as surely as data ghost of River is - that her mum and granddad have to build for her. I don't like River's fate either, but at least she's with friends and has the entire universe at her virtual fingertips. Rose's fate is, of course, much better than both of them, because she's with family and has a more than fair chance of making a good life, even with the complicated hand (pardon the pun! *wink*) she's been dealt.
Rose, and Martha and Jack and Sarah Jane still have chances to reach their full potential. They have the Doctor, and the memory of him. Their minds have been expanded by their experiences. Donna has less than she started with, because she started out with possibilities. It's not just the Doctor and traveling with him that's been taken away from her. The Doctor represents knowledge and the whole wide universe out there. Donna is now permanently denied knowledge of so much. She was a Time Lord for a few hours; she loved it, she wasn't grumpy and emo when she saw everything he normally sees in his head. She's gone from knowing it all to knowing less than most of us. Her world has been made very very small indeed. Wilf looking up at the stars on her behalf is no compensation at all for that. The stars might as well have gone out for Donna. The Darkness is right there in her head.
I know Donna's story's meant to be tragic - that's she's reduced to this - and it really did make me cry, which was what it was supposed to do... but I hate it. I utterly hate it. She shouts at the world because she's trying to be noticed and because she doesn't believe she's brilliant. They just made her even more invisible and more insignificant, and that IS a fate worse than death if that's not what you want.
The message I keep hearing on in Donna's story (and River's too) is that, rather than go out in a blaze of glory saving the universe and knowing how great you are, it's more important to just be alive... even if that means never really believing that you're wonderful or being able to live up to your full potential. You should accept something less than you are, were and could be, just so that you can cling on to a semblance of life.
Ironically, this is the opposite of the message of Life on Mars, where we were shown that it's better to accept the inevitability of death (to not be afraid of embracing it, to run at it and take a leap into the unknown passionately) and - in the meantime, before whatever happens in death happens - grab for yourself a blindingly brilliant slice of the life you really want. Accepting and making peace with the reality of death means choosing to really really live before you die.
It's more than just Donna and her tragedy. This "Everybody lives at any price!" business reduces the Doctor to someone who's just so totally shit scared of death that he'll actually hurt people to make himself feel better (although he'll be all tortured and angsty about it). I have no problems with the Doctor being a wanker or not a nice person sometimes, but this thing of having him force his fear of death onto others is becoming abhorrent. If nothing else, it's not a good message to be sending to the kids watching out there.
"People die and that's just the way it is, but it's okay sometimes because it's what you've done with your life that matters" is more honest and much healthier. They managed it with Tosh and Owen on Torchwood and, in the previous ep, they even did it with Harriet Jones (although in such a way that seemed to imply she might not be dead - and half fandom didn't believe she was and then didn't seem to care much afterwards because all the other stuff eclipsed it), but they're going off in some nasty direction with the Doctor.
I mean - a possible symptom of it - look at the way the fandom reacted to the Doctor regenerating. "We can't live without David Tennant as the Doctor! Panic!" Um, yes we can. I adore him, but yes we can. This show has survived so long because it's been accepting of change and being able to move on. Sometimes people leave. Sometimes people die. The fandom is all "Who, that's supposed to be dead(!), is coming back next!" It's unhealthy! Come on Doctor Who writers, teach us how to live with change and death instead of this depressing emphasis on bringing back the past (I'm counting all those bloody Daleks!) and on life at any cost, which devalues life rather than enhances it! Either flat out bring back Gallifrey and give the Doctor those challenges back or give him something new to distract him from the endless non-acceptance of death!
Even if you kill people off gracefully, the Doctor can still spend the last few mins of the series looking a bit emo (look at Jack, Gwen and Ianto!) if they really insist that he can't celebrate a persons life, instead of being a grumpy old self righteous martyr (with really great hair) just before the end credits.
I'm thinking that all RTD's cadre of writers had pets that died when they were kids and they've just never gotten over it. I'm really quite happy after this episode that they actually managed to kill off Tosh and Owen so beautifully and didn't pull some extra bollocks on that too! Although they technically did with Owen, and let's not even go there with Susie and Jack! Oh, and they killed Rhys too at one point, didn't they?! Issues. Definite issues.
Oh please, don't let it turn out that Tosh and Owen somehow aren't really dead!!! *horrifies herself utterly* Oh no, please no...