Since I read Apocalypse World, I read a LOT of different systems and also played some of them. :-)
In this way I think Ironsworn is really a milestone in the development of the of the pbtA-system. Shawn Tomkin manages to put the moves together like gears in a clockwork, focusing on getting rid of all the parts that are not necessary to create an ongoing plotline in a cold fantasy-iron-age-world with a strong focus on social interaction. I this way it is closer to a fantasy-Apocalypse-World-setting than Dungeon World with more serious drama and less Quentin-Tarantino-style action.
From the start of Apocalypse World there was this idea to play to find out what happens. Normally there still is a lot stress on the GM (my feeling) driving the plotline of the group, as (my) players are used to waiting to get a hind on what the next step in the adventure should be.
The idea of the Iron Vow as a basis to go on a quest is great. It is followed up by specific moves, if the PC decides to abort or fulfills it tying it back to the social world. Here a GM can really concentrate on the role of a facilitator.
With the Progress-System there is a really interesting push-your-luck-mechanism that is used for example for journeys, combat and personal goals of the PCs.
I also like the incorporation of the Mythic-RPG-way of creating new plot developments through open and closed questions and rolling on the probability. In this why a real ongoing co-op game in pbtA seems possible, which is not to forced to be a one-shot like Fiasco and other systems that are structured in this way.