About the Doctor:
"The Doctor went through hell, on Mars. I think its important to note the angry, vengeful Doctor glimpsed in that climax is gone-that was the point of Adelaide's death; she saved the future, and saved the Doctor from himself. He was brought back to his senses. And as you've seen from the children in need clip, a lot of the old, chipper doctor has been restored. Which is only right and proper, because that's the Doctor we want to see in his final story- the classic hero, the good man, the lovely funny, skinny fell in the suit and trainers... and as ever, there's a melancholy beneath the smile- he's still aware that he's facing the end of his song, and the ominous four knocks. He can't know whether this means regeneration or actual death - and neither can you, cos we might have some tricks up our sleeves!"
About both the Master and the Doctor and their relationship to each other:
"In 'The sound of drum's and 'Last of the Time Lords', he mirrored the Doctor by being powerful, suave and megalomaniac. Now, its new opposite - lost and raw and savage. Something's gone very wrong with his return - you'll have to watch to find out what! - but that x-ray effect see in the trailer, where he's all skull and bones, is the just the start of the trouble....imagine all the pent up forces of a Time Lord body - artron energy, regeneration energy, all the stuff - ripping open and broken loose. A dying timelord is a terrifying thing! So with the Master dying, the Doctor's end approaching, and both determined to survive, they're hurtling along an almighty collision course. And that's just part one!"
"Donna had all her memories of her time with the Doctor erased. If she even gets a glimpse of the Time Lord again, the pressure on her mind could kill her..all those strange, powerful forces are still pent up within Donna's head, but what would happen, if those energies were provoked?..Donna faced all sorts of Christmas disasters but this time, its more personal and more deadly then ever before."
Also, there's quite a few more spoilery information given on the blog. Go read it all here.