This is a very fun episode overall.  It's tense and has great atmosphere.  I love the ghosts and mystery around it - especially here, in the first episode, we're given a lot of interesting question and setup for the next part Before the Flood.  Under the Lake would probably be a classic but the writing around the Doctor and Clara is a little off here, sadly.  The whole scene with the Doctor reading from cards felt a little awkward and too far for his character.  Yes, the Doctor can be weird in social situations, but parts of this episode go far in almost making it seem like he can't function in any situation.  It just feels a little off in some regard.  The Moffat era kind of went in this direction with the Doctor's character that sometimes works and can be funny but in episodes like Under the Lake it just doesn't feel right for whatever reason.

It's complicated.  It's interesting.  I think the core of it is that there's a lot more to be said for the autism coding in Moffat's writing.  I do think it has its pros and cons, but here is a good example as to where it has limited effectiveness in terms of solid character writing as we slide into tropes like "my autism is my superpower."  Which, to be fair, isn't directly being said by this episode, sure, but it is kind of in between the lines and though I do very much like this episode, it only rates so high because of more systemic issues within the writing that are very much hard to ignore here.

The most traditional the Capaldi era ever got, but this isn't necessarily a fault. The show knows it's traditional and it also knows it doesn't have that much of a knockout in terms of monsters or twists. So the focus becomes character work. This is an expertly characterized extended cast of characters. You really love the extended cast, and it hurts when they're whittled down. It's the oddest thing. In the right hands, you don't need fresh new ideas to be effective.