This was a really good episode, and for once I actually found something in this show creepy, that is the hospital. Actually, that whole Mondasian world that was created was very atmospheric and moody, with a sort of depressed feel to it. Really well done.
It was cool to have the Master doing his disguises again, although I instantly recognized John Simm anyway. I'm actually disappointed they made it public he'd be in this episode rather than playing with a James Stoker style alias in the promotional material. Hey, we're doing old school Master stuff, might as well go all the way, right? I really hope part 2 explains how he survived TEOT and somehow cured a body that was barely holding together without regenerating, and has kept this form for what is implied to be several years. And is it me, or did the Simm Master's disguise remind anyone else of a Klingon?
The flashback scene showing the Doctor talking about his friendship with the Master to Bill at the university was also really well done. Especially with the new variation of This is Gallifrey that played through the scene.
I'm very pleasantly surprised to see the one Moffat trope I have loathed throughout his run so wonderfully deconstructed: the Moffat Monologues! When the Doctor is trying to talk Blue Man out of the gun, he begins what sounds like a very lengthy and Moffat-esque monologue, and Blue Man instantly fires and kills Bill. Has Moffat finally realized how awful those monologues were, perhaps seeing even more excruciating monologues on Class has finally convinced him enough is quite enough.
Finally, I worry they might be trying too hard with their Tenth Planet homages. As great as it is to see the Mondasian Cybermen, and I really dig the whole thing about they're always screaming to explain why their mouths moves the way they do, the opening scene with the TARDIS in the snowy wasteland overdid things a little by framing the scenes almost exactly the same as the exterior shots of the TARDIS just before Hartnell's regeneration scene. That's pushing a homage a bit too far, almost going in territory STID went with copying a certain scene from TWOK.
Meh, that boat already sailed in November 2015 when the twist ending cliffhanger of Heaven Sent (the Doctor was on Gallifrey, the Time Lords were the ones after the Confession Dial) was released officially through the BBC several weeks in advance. Whatever, some people are way too protective of secrets these days, such as again, STID which sat on the identity of Cumberbatch's character way too long and went to embarrassing lengths to keep it a secret, that I find someone who'll officially give away spoilers quite refreshing.