Why is this appealing?
You mean, you don't find walking your haughty triangle body around honking at people who hate you fun? Suspicious, but fair. Described by its developers House House as a "slapstick-stealth-sandbox," Untitled Goose Game is basically just a puzzle game that's more sophisticated than it lets on at first glance. Each level's tasks are deceptively complex, and the difficulty jumps fairly significantly from one area to the next. For example, that "have a picnic" job in the first area means you need to collect a laundry list of items and carry them in your beak to your picnic blanket without getting caught by the groundskeeper, lest he take your loot back to where it came from; when you reach the next part of the town, you need to think about how to use your environment and its seemingly random items to trick the dumbass neighborhood kid into the phone booth so you can lock him in there, or untie his shoes so you can nab his glasses off his face. And so on and so forth. If you're a "gamer" who "gets" references, you'll find parts of recent titles like Baba Is You and Donut Country in here.
There is a perverse pleasure in doing all this bad stuff as a storybook cartoon goose bastard, which is heightened by the clever in-game details. The horrible goose holds itself like only a shitty goose would, staring down at items it dropped from its beak with the same energy your bad, but playful, dog or cat would if it dropped something it knows it wasn't supposed to be messing with at your feet. It stares back at the people nearby who so blatantly don't want this awful goose around as if it got off on being in places it's not supposed to be. In progressive levels, you can leverage items like glass milk bottles or traffic cones to alter the timbre of your honk, or use walkie talkies to scare villagers (heheheheh). Even honking and flapping your wings just to wreak havoc or scare the dumbass village boy brings an impish joy.
Lastly is the delightful and sparse reactive soundtrack, adapted from Claude Debussy's Preludes, as The Verge reported. A solo piano twinkles softly when the bad goose is thinking about getting up to no good, and hits in full chaotic force when the bad goose starts a conflict with a human that doesn't want it around, or gets caught stealing something it shouldn't have.