The first few stories have many underlying concepts running through throughout them which will hopefully coalesce into a grand Christmas special. First is the concept of immortality. Davros wanted it so much that he canned his entire civilization so they could live forever. Next we have the ghosts, which are left at the end of the story as lost souls trapped forever. Of course, we get the immortal Viking demonstrating the loneliness of living forever. Will the Zygon invasion end up with Osgood or Clara in a sort of immortal state? Could be. 
The next idea we see is introduced by Davros as a mysterious hybrid. I can only assume every story will include one in some shape or form up to the end--Ashilda, Captain Jack, Osgood...I think we all know who the ultimate hybrid is though and she is waiting at the end of the season. I have enjoyed the flashbacks as they have been fixing so many of the complaints I made about last season. We return to Pompeii to explain the face. We return to Mars (not in true flashback, but under the lake was very similar to "Waters of Mars") to see his attitude change about rules. We return to the 3 doctors to see the beginnings of the Zygon invasion. I can only hope this continues, as it shows how interconnected the Who world can be at it's best.
While we are on the subject of the best of writing, let's address last season, which I really didn't like at all. Through most of it, the Doctor was written as a supporting player, begging for Clara's affection. Whether putting down her boyfriend or just trying to impress her constantly, he came off as an ex-boyfriend who had been dumped into the friend zone, being kept around in case the new boy didn't work out. This season there is no doubt that he is the lead. In fact, I would argue that the roles have been reversed, and that he is now a sort of a "booty call" for Clara (not in the true sense of course). Many times it has already gotten to the point that he has told her to leave him alone, basically.
