I'm fascinated by Apocalypse World and its progeny (i.e. Dungeon World, Monster of the Week, Apocalypse Galactica, The Regiment etc.).
The core rules of the game are easily accessible. The tone and scope are encoded in the moves…and each table is encouraged to create custom moves appropriate for their session/game.
The games mandate that lots of questions be asked and answered – after all everyone is there to play to find out what happens. This strongly discourages isolated world building, and instead pushes towards defining the world through play.
But, most importantly, my fevered fascination with the game comes from the well defined Failure, Partial Success, and Success of a move. Certainly this isn't the first game with three (or more) possible results for a roll. But it is the concrete definitions of Success and Partial Success that ignites my brain.
Another interesting thing, when I've GM-ed one of these games, I rarely want to see a move Fail. After all, Failure means that I can make a hard move. I am on the hook to come up with a response on the spot. In this way, the games naturally steer me towards “Being a fan of the characters” because the other path is more work.
Contrast with a Partial Success for a move. The Partial Success of a move provides a framework to work within. Even with a set of well defined GM moves, Failure is more involved than the other two outcomes.
That isn't to say I won't do my job and make a Hard Move. In fact, the hard moves are my time to shine and inject more into the story. Separate them, dissolve their sword, bring in reinforcements, and in general push them hard. This then makes their success all the better.
Another tangentially related reason for my fascination is that the games are very quick to pick up and start playing. The moves are right there for everyone to see. Each game has different conflict resolution systems – Dungeon World's the least like the others. So I can quickly pick up each of these games, knowing that the Agenda, Principles, and Moves are the Cliff Notes of what the games will be about.
Thank you Vincent Baker, and the entire Apocalypse World hacking community. You've created some great games that I look forward to playing.