It's a hack of Apocalypse World for D&D style dungeon romps. The DM never rolls dice, player actions are a wide choice of simple attribute checks with many "three outcomes, choose two" results. Neckbeards like it because it has the feel of old school AD&D in that indie-RPG style.
Your standard D&D classes are here, and characters have Str/Dex/Con/Int/Wis/Cha stats. However, you don't get to roll for them in the default rules, and you use the given array to start. Attribute bonuses are the -1 to +3 from most D&D editions. The stuff players can do in any situation are called "moves" and each has an attribute associated with it.
For a "move" the player rolls 2d6+bonus to determine success; 7-9 is "success with consequences" and 10+ is total success. On a 6- you get XP, which is one of the interesting parts of the game.
The "consequences" of a 7-9 depend on the move, and are usually vaguely worded. and the player usually has a choice of what the cost will be. (E.g. The Fighter wants to punt a goblin into a pit. This counts as an application of the 'hack and slash' move, as the PC is trying to physically interfere with or hurt something, even if it's not actually hacking and slashing. The Player rolls a 7, success with a cost, so the GM proposes this offer - the Fighter can knock the goblin into the pit, but in doing so, the fighter takes damage grazing the goblin's sword. It's then up to the player to choose which outcome they want - goblin in pit, or remaining undamaged.)
The DM has their own list of "moves," but no stats, and never rolls dice. That's okay, since players can always roll dice to resist, avoid or rejigger stuff that would hurt them. The moves are never mentioned by name, it's more to organize what the DM can do to the players. The players' actions and declarations drive what happens in the adventure, with the GM's promptings providing interesting choices which influence but need to direct the players.
The upsides are:
- It's a nice rules-light game with a kind of neat core mechanic.
- Marking XP on a miss is sorta cool.
- Quick and easy to play.
The downsides are:
- All monsters have the same TN to hit. Yep, whether it's a will-o-wisp, or a fat-ass gelatinous cube, it's the same difficulty to land a blow. Though you can easily make a monster harder to hit by giving the players -1 to hit that monster.
- Not really a lot in the way of character options. The "moves" you gain are set up almost like 4e maneuvers, in the way of "pick from a buffet as you go along" instead of making a character from scratch.
- Still has many of the pitfalls of D&D, like alignment, though alignment has more function in Dungeon World as you can use it to gain more xp.
- Race-class restrictions. Sure, you can homebrew your own races / classes, but why couldn't the book just do it right in the first place?
- When you return triumphant and throw a big party, spend 100 coin and roll + extra 100s of coin spent. On a 10+ choose 3. On a 7–9 choose 1. On a miss, you still choose one, but things get really out of hand.
- You befriend a useful NPC
- You hear rumours of an opportunity
- You gain useful information
- You are not entangled, enchanted, or tricked
- Cast (Wizard)
- When you release a spell you’ve prepared, roll+Int. On a 10+, the spell is successfully cast and you do not forget the spell; you may cast it again later. On a 7-9, the spell is cast, but choose one:
- You draw unwelcome attention or put yourself in a spot. The GM will tell you how.
- The spell disturbs the fabric of reality as it is cast; take -1 ongoing to cast a spell until the next time you Prepare Spells.
- After it is cast, the spell is forgotten. You cannot cast the spell again until you prepare spells.