Well, not really, as almost all of the ongoing mysteries have been revealed. There's still some goings-on concerning the "something" on Donna's back, but more on that below.
So the Doctor is regenerating! Well, no, because Tennant's in the Christmas episode with the Black Cybermen and David Morrissey, so what is going on? Clues...
There was a very conspicuous close up on that severed hand early on in the episode, and one that served no purpose. Yet.
And when the Doctor was shot, the "extermination" visual effect only covered a quarter of his body, not the whole thing as it does with others. That quarter just happened to include the arm that matches up to that severed hand.
So I reckon that only some of the Doctor's body (including only one of his two hearts) was damaged by the Dalek, that his regeneration won't take, and that he'll be caught in some kind of mid-regeneration, Ten-and-a-Half, state. The process will be happily reversed by the end of the episode by somehow using that severed hand to replace the dying bit of the Doctor.
How the lingering plot threads regarding Donna will turn out, I have no idea, but I do wonder if she'll have anything to do with fixing the regeneration. There's also a suggestion that she still has one of those time-bugs on her back, so perhaps she's going to change history again; if they use that to turn back time once more and undo everything, that'll be annoying.