I try not to overthink, but when you're doing nothing but lying down with a bunch of tubes shoved up your ass....
I do understand that regeneration is not the most desirable outcome, but this is a man who faced off against Daleks, Cybermen, statues that make you live to death, Jedune, and a dozen other threats, facing that same threat of death without this kind of response. And that's just the tenth Doctor.
And regardless of multiple personalities, Eccleston's Doctor did face it with a smile. Possibly because he wasn't alone at the time, but still, he knew he was "dying" and another personality would be there. And Smith's Doctor walks off stage right, knowing there's a chance he doesn't simply regenerate, but ceases to exist forever, and he does it with grace. Subtle, somber grace, but still grace.
I do think those are the main reasons it sticks with me. The Doctor doesn't seem afraid of death most of the time. Suddenly, he is. Even at his darkest moments in the new series, he faces down the worst threats time and space have to offer.
Unless they knock four times....
I do understand what you're saying that it's painful, that another personality rises up, etc., but again, that threat has always been there. Throw in the fact that Nine "died" from a similar sacrifice, absorbing the radiation of the Time Vortex to save Rose from burning up as opposed to walking into a radioactive death chamber to save Wilfred. Frankly, I think Nine should have let Rose fry, but I'm not RTD.
But the thing, the main thing, to compare the two: Ecclestone goes out joking about Barcelona. Irreverently talking about how he could have two heads or none. Telling Rose she was fantastic. And how he was, too.
Tennant goes out whining that it's not fair. That he could do so much more, that he dosn't want to go. He borderline derides Wilfred for such a human act, the very sort of thing the Doctor normally loves about humanity. Granted, he doesn't want to get in Wilf's pants, but still.
That one line overshadows everything else. "It's not fair." It overshadows "Wilfred, it's my honour." It overshadows "I've lived too long." And it continues, because right to his last words, the Doctor laments it rather than coming to terms with it.
Not saying he should love it, or look forward to it. Not saying he shouldn't even be attached to his current persona. I just wish he had gone out with more dignity. I mean, especially after the high note that he's taken care of his past companions.
And like his speech to Wil, the "I don't want to go" really overshadows that.
I did like series five a lot. I've watched it like four times since the DVDs dropped, and I'll probably watch it a lot more. I think it was probably more solid than any other single series since the reboot, at the very least. And Smith could be better than Tennant, though I think the fans will never give him the chance. Well, I think a good chunk of the fans won't. Tennant is huge and it's unfair for anyone to really follow him, because they won't get the level of credit they deserve. Actually, I think Eccleston is criminally underrated too, and gets overshadowed because the very next actor was Tennant.