As non-spoilery as I can be behind the cut.
Part of the coolness of this episode was ruined by my husband, who, while we were watching the prologue, said, "Oh, that's (person name)." He had no doubts about it, and thus, the reveal in the episode was not shocking like it should have been. Damn it, man. You with your perspicacity. You rumbled me!
The scenes with (person name)'s henchmen were great! I didn't catch his name, but he was rather ingenious and beautifully designed.
I also very much enjoyed Missy here. Unlike her last appearance, this time she showed actual intelligence (albeit insane intelligence, which is characteristic of the Master) and acted independently of her new obsession with the Doctor, and she worked on the same level as the Doctor, showcasing their best friend/best enemies dynamic.
Clara was still annoying, though again, not as bad as last season. I see we're still on the "lying is bad" kick. The scene in the square was fantastic.
Of course, the thing that I'm most concerned with is how this is going to fit in with continuity, but that's something for the next episode to resolve. There's also a disturbing implication in the very last scene which I won't talk about because of spoilers, but again, we'll see where it goes next week.
Last couple of things that I didn't like, mostly continuity-related and canon-related... I see UNIT is completely useless. Oh, something strange is happening around the world, but we can't even start to look at the problem until we call in the Doctor's companion. Just wow. At least Kate appeared on the screen without being a vehicle for referencing the Brigadier for once, but she's still incompetent. And, lastly, just like Eleven, Twelve doesn't mind rampaging through time and introducing anachronisms. (As a side note, you ever notice that the Doctor never messes with the history and timelines of any other planet? I'm starting to wonder if he loves Earth as much as he says he does... ;)