Dungeon World provides GMs with great advice on how to run a good session.
Additionally, GMs can set conditions that, if fulfilled, will give a player XP. It gives the GM a way of enticing players to perform certain actions they might not otherwise have considered or wanted to perform.
Relationships between characters are established during the character creation process as well as at the end of each session via Bonds. Bonds are simple statements that explain how a character relates to another character. An example of a bond would be, "Avon proved himself a coward in the dungeons of Xax’takar. He is a dangerous liability to the party and must be watched." Multiple bonds can be established at a time. At the end of each session bonds resolved and new ones created give XP.
Unlike other RPGs where the GM creates the world and the storyline single-handedly, here GMs are specifically asked to leave blanks and players encouraged to fill them in.
XP is given at the end of the session if a character is acting according to alignment. Alignments are categorized as Good, Lawful, Neutral, Chaotic, and Evil.
Dungeon World is set in a D&D-esque world with a distinct slant towards combat and treasure gathering while emphasizing narrative over combat mechanics.
The game includes classic fantasy RPG classes such as Bard, Cleric, Druid, Fighter, Paladin, Ranger, Thief and Wizard and D&D style ability scores such as Strength, Dexterity, Constitution, Intelligence, Wisdom and Charisma.
In Dungeon World, XP is given for failures, and automatically for any rolls that are 6 or below. It provides many benefits and chief among them is the fact that it allows players to catch up to the difficulty of challenges faced. Additionally, it encourages the group to attempt harder challenges, reduces grief caused by failure and clears the GM from having to track each individual player's level.
The text is CC-BY but the game as a whole (graphics etc) is not free. You can make and sell your own games based off it but you won't find the PDF download for free anywhere.
Dungeon World's combat system is not divided into turns in order to make it more fluid. But it can get confusing as to what happens in what order. It is purposefully frantic, but that can turn off some. Especially people with preconceived notions of how RPGs should be played.
In the most basic form - GMs narrate the beginning of an NPC's action to which the player responds. If the player's response triggers a move, roll the dice. GM narrates the conclusion.
Fate uses "aspects" to influence the gameplay in ways that are consistent with the world that's built. Aspects are descriptive phrases that define anything they are attached to. They give both roleplaying and mechanical advantages and disadvantages. For example, a character could be "raised in a wretched hive of scum and villainy" and that would allow him to better negotiate with thiefs and crooks, but also make him poor at fine dining etiquette.
Whereas Fate Core is skill driven, has more variables to keep track of and is potentially more varied in terms characters created, Fate Accelerated is approach driven, has less variables to keep track of and allows getting started quicker.
Also, the Accelerated version is just 50 pages whereas the Core version is 310 pages.
It starts at the very beginning by explaining what is roleplaying and moves on to introduce various mechanics and conventions and explains why they're there, when's the appropriate time to use them and what are the best practices on how to do it.
The book does so in easy to understand language, offers many examples and highlights the important bits in various ways.
Compels allow GM to trigger a player's aspect in exchange for a fate point. The player can avoid invoking noted apect by spending a fate point.
Concerns about the system include removing control from the player of how a character is played, GMs having a lot more data to track (each aspect for each player) and possibility of getting stuck in a compel-loop of forcing certain behaviour.
Coming up with aspects can take a while. Especially for new players. In addition to coming up with traits that define the character, tags and compels are needed. Furthermore all of these little nuances have to be coherent as well as balanced in terms of power. This can make the character creation process take a while.