I love this game's main mechanic! It's simple and elegant, while still allowing characters to specialize in a way that sets them apart from each other. Setting the default DC low helps make players feel powerful when they use their characters' specialties, while still giving an untrained character a decent chance at success. It seems great for introducing new players to roleplaying, too.
If I had any complaints, it would be the use of HP for monsters. Some action types seem like they would deplete HP (Fight), and some don't (the examples for Sneaky and Knowledge), and the book doesn't give the GM any guidelines for what to do if a move doesn't deplete HP. I can imagine an alternative system with something like "this monster takes 5 successful rolls to defeat" instead.