Border Patrol: The robot torpedo shark that patrols the waters around Mafia Town and keeps you from swimming out too far. Also serves as a kind of Broken Bridge, since it forces you to buy boat tickets to visit the outermost islands around the town.
Boss Subtitles: Only one as of Alpha, but the battle against the TNT-toting Mafia members gets a subtitle.
Bratty Half-Pint: Hat Kid shows quite a bit of brattiness in certain parts of the alpha (like when she angers a mafia by sticking her tongue at him).
Cool Mask: All of the Forest Dwellers wear animal masks (possibly because they have no faces otherwise). Hat Kid can even wear some of them herself.
Everything's Better with Princesses: Once upon a time, Queen Vanessa was Princess Vanessa. Then she was crowned queen... And thanks to the loss of her cute, sparkly title, everything went downhill from there.
Evil Brit: Mustache Girl, at least when she has proper voice acting.
The Family for the Whole Family: The Mafia of Cooks. Their criminal activities involve beating up barrels dressed as old ladies (to intimidate people) and punching seagulls for eating their fish, and they won't harm Hat Kid unless she attacks them first.
Gainax Ending: The ending of the Alpha Build. After completing the two storyline missions included in the game, you get a nice "Thank you for playing the alpha build of A Hat in Time!" message. Suddenly, you find yourself in the middle of a dark, snowy street somewhere in what appears to be France during the early 1900's. One of the storefronts opens, and out steps what appears to be Hat Kid... Only she's very much not a kid anymore. You can walk around the quiet, snowy street corner, but you can't jump or use any of your abilities. Eventually, you can walk up to the window of a bookstore, which appears to be displaying several books of fairytales, to see the credits.
Guide Dang It: One of the Mafia Town hourglasses requires you enter a pipe & complete an obstacle course like the ones in Super Mario Sunshine. It's a really small pipe in a very remote location. Somewhat helped by the fact that there are some orbs inside it, and the fox mask (apparently) glows green when near secrets, but there's still the problem that it's tiny and remote.
Shock and Awe: The Electro Pin, which allows you to damage enemies who touch you directly.
Shout-Out: Some of the graffiti in Mafia Town reads, "Come and have a go, if you think you're hard enough." While this is a well-known chant associated with Football Hooligans, given the game's inspirations (and the fact that one of its composers is Grant Kirkhope), it's almost certainly a reference to Banjo-Tooie and how Mayahem Temple's music is a corruption of this chant.
The announcer for the balloon race in Mafia Town is doing his best T.T. impression.
Using the Fox Mask in one area of Queen Vanessa's manor reveals a hidden drawing of Raz.
Taken for Granite: Queen Vanessa's gaze turns her victims to stone. Her mansion is even decorated with several of her... trophies.
Toilet Humor: There's a haunted outhouse in the second level. The haunted toilet is apparently so menacing it gets its own Leitmotif.
Variable Mix: Used in a similar fashion to Banjo-Kazooie, with the instrumentation of the level theme changing in different areas.
Video Game Flight: Hat Kid can jump on firecracker rockets to fly. Most of them have a limited fuel supply and will kick her off rather quickly, good mostly for getting a good look at the level, but you can use one with a much larger fuel supply to race a Mafia goon with a balloon.
Yandere: Queen Vanessa. Reading her letters and diaries around her mansion reveals that she freaked out when she was crowned as queen because it meant she couldn't be a pretty princess any more, and she apparently cut off all her lover's hair in his sleep and dyed it so it would match hers perfectly. She also banned bacon from the house because she thought he loved it more than her, and has apparently had thoughts of chaining him up in the basement so he can't escape.