Let me just say thank you--this is the season I have been waiting for! We are tying up loose ends, we have a real story arc and we don't have a romance between a thousands of years old man and a twenty something girl. Thank you, thank you, thank you.
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.
In closing, here's where I hope the season is going. We will continue to see red herrings brought in to divert from River. At some Point, the good captain will return an we will find out he has been keeping an eye on the girl that can't die (casting the "voice of Bo" as her caretaker can't be an accident). We will finally get a huge season ender when the Doctor gives River his sonic in the last scene to mirror the first scene of the season when he gave one to Davros. Zygon Clara becomes the girl in the library to protect river and we have a beautiful end to both of their stories.
We finally get back to a single part story. I kind of missed them and was sure glad this was not a 2 part story. Talk about a snooze fest, which is kind of funny since this episode is about sleep. Leave it to Dr Who to take eye gunk and turn it into a monster. Which I have to admit kind of looked cool, but the idea that these creatures are made out of sand in the corner of your eye, left me wondering if Steven Moffat ran out of ideas. This whole episode kind of felt like a filler after the Zygon episodes.
I try not to be political at all on this page. I have seen politics ruin long time friendships on Facebook and other platforms. With that being said, I apologize in advance if this seems political, but after watching these 2 Zygon episodes, I couldn't get this out of my head. I want you to look at all the Zygons as Muslims here on Earth and the rebel faction of Zygons as ISIS (or radical/extremist Muslims).
I can't even really discuss the actually villains, because they were never really explored as characters. I can't even remember their names at all, it was that underdeveloped. I know the point of both episodes was to show Lady Me, or whatever she calls , but it would have been nice to know a little bit about the villains. Then again, it didn't really think there was much to these villains. One of them looked like a space pirate and the other looked like the beast from "Beauty and the Beast".
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. 
Watching "Under the Lake", I was really impressed with the creepiness of the episode. I mean the ghosts are really creepy with their eye all blacked out. You know this is 2 episodes in a row now that I have been creeped out over eyeballs. You had the the hand mines with their eyeballs on their hands and now these ghosts with no eyes. With that being said, this first episode is a great mixture of what makes Dr Who great. You have a fantastic mystery, paranormal activity, science fiction and it's all mixed under a lake.