(This post is about the core Burning Wheel mechanics. I'll make specific posts about Burning Empires and Mouse Guard later.)
Burning Wheel really turns roleplaying on its head in many ways, and since I picked it up, it's really changed my views on both storytelling and game mechanics. I love it and hate it for the same reason; like watching something both fascinating and disturbing, Burning Wheel sticks in your head and forever alters your perceptions. Picking up and experiencing Burning Wheel has forever infected my view of gaming and I regret nothing.
On the face of things, Burning Wheel uses a straight-forward dice pool mechanic, but it's Burning Wheel's conflict engine where things really shine. BW is both really open about story-mechanics, yet ruthlessly competitive when it comes to conflict mechanics and the consequences of choices. Also, and perhaps more importantly, Burning Wheel makes failure interesting (Mouse Guard especially pushes this concept).
Burning Wheels conflict engine uses a core concept of secretly scripting actions in advance, then revealing them one at a time and resolving them rock-paper-scissors style. Attack versus Defend? Whomever rolls higher succeeds. Attack vs. Attack? Both sides could succeed or fail, independently of the the other's success or failure, and so on.
Burning Wheel fantasy has three types of conflicts: Fight!, Range-and-Cover and the Duel of Wits. While Fight and Range-and-Cover clearly deal with martial battle, the Duel of Wits covers social conflicts. Both sides in a DOW start with a pool of 'Disposition' points, representing their initial position and strength of argument. Exchanges of Points, Rebuttals and Obfuscations whittle away at your opponent's Disposition. The side that looses it's last Disposition point looses the argument! The winner, however, may have to make concessions based on how close to zero their Disposition was brought to zero! The essential conflict mechanic scales well, and allows for narrative control over the context and feel of the story.
Burning Empires, the sci-fi BW setting, adds several more conflict types: the Firefight (Range and Cover writ large and with a more strategic point-of-view) and the Infection-level campaign conflict which handles events of planetary importance.
Mouse Guard introduced a streamlined version of this core conflict engine concept, which allows for settling just about any form of contest or conflict, with only four 'move' options (Attack, Defend, Maneuver and Feint). Which skills are tested for each Action depends on the nature of the conflict. Adding new conflicts is easy: simple define which skills are tested for each Action.
Unlike most other RPGs -which traditionally require a ton of creative world-building ahead of time on the part of the Game Master/Storyteller- Burning Wheel thrives on starting with simple world concepts and filling out the details and world facts through character creation and in actual play. Instead of asking "What do I know about (X)?" players instead say "My character knows (X) to be true," and then rolls to see how right/wrong they are. I love this idea, because it enables player and character investment in the game and the world, which makes for richer stories and deeper attachment to character and world.
Character creation is a process of choosing Lifepaths that affect the character's age, skills learned and traits acquired. Want a skilled, experienced character? Then accept that they will either be older, or heavily affected by the tough life experiences they've gone through to get that good. You character will pick up traits and habits that may seem negative or inhibiting, but these play into Burning Wheels drama economy (called Artha), you want complicated and challenged characters because their flaws and failures actually fuel the Artha economy. The Stocks (races) and Lifepaths available to a campaign also feed the world narrative and help to define setting.
Circles, which is a social attribute, is how well socially connected your character is. Instead of taking fixed contacts or allies before play begins, you can test your Circles to see if you can find someone with the skils/knowledge you're looking for. Failing that test doesn't mean you don't find them; instead they may hate you, or some other side-quest your character needs to meet. Circles is a brilliant idea and I've considered adding to other game engines.
Character Advancement is based on testing your character's abilities. There are no Experience Points, no Levels; killing one hundred orcs doesn't automatically make your character better at cooking. Want to improve an aspect of your character? You have to test it and push it beyond what's safe or routine. At higher skill ranks, you have to try things that are patently impossible to succeed. This is where the Artha economy really helps; you need the help of Artha to improve abilities beyond a certain rank. Tracking the number of tests for every skill and ability is a little taxing, but you only count the ones that actually count towards advancing that skill.
On the downsides, Burning Wheel is very different, ruthless and character-lethal system. It's not difficult to get your character maimed or killed in the fantasy and sci-fi settings that have thus far been published. Mouse Guard is different, but character death is still possible. The essentials of Burning Wheel's conflict system(s) are challenging to grasp at first, especially to old-school gamers who are traditional in their experience with gaming.