I am going to let you in on a secret: the Mouse Guard Roleplaying Game is the only RPG book I have read all the way through, cover to cover. In fact, I have read it twice. That speaks to two things. The first is that the game is different enough from other RPGs I'd played that reading everything was necessary. And the second is that the book is written so well that it encouraged me to keep going--usually the dry textbook-like prose of roleplaying manuals leaves me bored and fatigued from all the minutiae and rules upon rules, leading me to eventually skim at best or give up at worst. This book I actually enjoyed reading.
Luke Crane and David Petersen have created an excellent and exciting thing with the Mouse Guard roleplaying game. Based off of Petersen's popular comics about a band of rodent adventurers and heroes, Crane designed the game to imitate the narrative structure and conflicts of the books--conflicts revolving around local politics, predators and the toil that comes with living as small, defenseless and easy prey in a world much larger and more dangerous than oneself. Crane succeeds spectacularly.
If there is one main accomplishment with Mouse Guard, it's the perfect matching of a game system to its source material. In most cases for licensed roleplaying games, designers will often take the basic assumptions of Dungeons and Dragons and fiddle with the details until it vaguely resembles whatever work they're attempting to emulate. Mouse Guard does not do that. Though based off of Crane's Burning Wheel RPG, which I'll note I have never played, the game is specifically structured to create stories and situations that would occur in the comics, and to resolve them in the game as they would be resolved in the books. This is illustrated in the rulebook itself, as Crane uses the events of the first volume of the comics as an example for how the game is run, showing how what happens in the story could have occurred in game, using the rules exactly. It works perfectly, and the plotlines Petersen developed for his characters could easily occur naturally and organically within the rules of the game.
As far as the rules themselves, Mouse Guard uses dice pool mechanics to represent and test a character's ability. Most tests are simple, with a player throwing a handful of his or her dice and counting up successes--any die that comes up with a number of four or better counts as a success. The difficulty of a given task is represented by its obstacle number, which a player must surpass in number of successes for the character to succeed. Sometimes players may make a test against an opponent, and must roll more successes than their challenger. More complicated tasks are done as conflicts, which follow a scripted, rock-paper-scissors like order. In a conflict, characters join together in teams of three or less--usually there are two teams: one for the players and one for any opponents played by the GM--and have a number of actions to choose from, including defend, attack, feint and maneuver. Each team secretly chooses three of these actions in order and then both teams reveal their actions at once, with each specific action type interacting differently. Conflicts are performed to resolve more than just combat, and are often used to represent arguments, chases and even navigating through treacherous terrain. Each team determines their goal at the beginning of a conflict, which can range from getting through a flooding forest to vanquishing an enemy. It should be noted that a mouse can only die in combat if that was his or her opponent's goal from the beginning, making Mouse Guard a game with potentially low lethality--not that death is unheard of, and not to imply that the characters don't face serious risks over the course of their adventures.
The main innovation rules-wise, and the means by which the game is able to so easily create stories and situations you'd come across in the comics, is the separation of a game session into a GM's turn and a player's turn, combined with the game's core assumptions about what conflict resolution means. Most of a session will consist of the GM's turn, which doesn't differ too radically from the standard set-up of a game of Dungeons and Dragons. The GM is in charge here, setting the situation from the characters, describing where they are and how they got there, and giving them their orders and describing the circumstances. From here the characters jump headlong into their mission, and for the rest of the GM's turn are at the whim of fate, bad luck and struggle to do what they must do. The mice are on a mission, and they get no relief until it is resolved. They can't turn back and they can't give up. The characters either have to succeed or fail, and that's where the genius of Mouse Guard begins to shine, because failure at a particular task is always a possibility, and is often likely. But that's not the end.
In a game like Dungeons and Dragons, the assumption is that the characters will succeed. The rules simply weren't made for them to do otherwise. And, if they do fail, usually it means death and the game grinding to a halt. In Mouse Guard the game keeps going. It's just another twist in the story. If you fail to cross the river safely, you're washed further downstream and perhaps lose some of your important equipment. Or if you lose a battle with a massive rattlesnake, maybe the important mouse you're supposed to be escorting gets gobbled up and one of the players loses a tail. The story doesn't end there. You have to deal with those consequences.
In the GM's turn the players react to the situation around them, as dictated by the GM, and must do their best in the face of the terrible odds placed against them. But in the Players' Turn, the characters take on the active role. When their mission is complete, everyone can relax and the characters decide what happens next. Perhaps a mouse will go and buy some goods, or visit a friend or lover. Or perhaps one of the mice decides he must settle an old score with a rival who lives nearby. When the players are in charge, it's the GM who must respond to them; each character gets to say what his or her character goes out to do, and the GM responds with appropriate tests to see what happens.
This dynamic sets up for pressing adventure and urgency while out in the field, followed by a more casual, player-driven counter narrative taking place when the characters return to town and set to accomplish more personal goals. It allows the game to mimic the narrative feeling of the comics, coupling epic adventure and triumph with equal amounts of personal character interaction and development.
As a whole, Mouse Guard does what it sets out to do almost perfectly. It is fun to play, recreates the tone and structure of the comics, and is set in a world very different than most other roleplaying games. One of my very favorite things about the game is the perspective from which the characters see the world, and how the world they inhabit--which really is the same as ours--can seem so strange when seen from a different point of view. These characters don't fight dragons or use magic, and they aren't on quests to save the world from ancient curses. They're just little mice trying to survive. And the world and its threats, which to us are all very mundane things--navigating through a spring rain, surviving a snowstorm, crossing a creek, driving off a herd of deer--are life and death struggles for creatures so small. Mouse Guard is unique as a game in that it lets you see and experience the world differently, and in the end that's something really wonderful.