These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crosses the Line Twice: In Chapter 17, Thor proclaims (on Harry spending an instructive half an hour with Tony and Darcy) that Lily will probably return from beyond the grave to strangle him for this. Cue Loki conjuring an illusion of an infuriated Lily. Thankfully, Thor takes it pretty well.
Crossover Ship: It's revealed that McGonagall was the fiancee of Bucky Barnes. It becomes more meaningful when it's revealed that she was pregnant when he "died," and reveals that Hawkeye is her grandson. Meaning that Clint is the grandson of one of the most powerful witches alive and the Winter Soldier, the world's most notorious assassin.
If you had any doubts about the identity of 'The Hidden World', as the author note mentions, translating 'The Hidden One' into Ancient Greek reveals the name of the planet. Krypton.
The title of Chapter 34 is both a reference to the insanity of Harry's Crazy Enough to Work plan and the name of a chess strategy which relies on acting fast, Refuge in Audacity and taking the opponent offguard. Much like Harry's plan.
Jerkass Woobie: The Disir. They may be villains, but you can solidly blame Bor and Malekith for what happened to them.