Dungeon World is the Roleplaying Game I've been wishing I was playing while I'd been playing Palladium and D&D years ago.
Dungeon World describes itself as a conversation, and this is absolutely true. The game doesn't differentiate between "Roleplaying" and "Combat", treating everything as roleplaying in a seamless flow as the players and GM talk about what is going on.
The game is very improvisation-heavy, giving the GM tools to decide what happens when the players either look to them to see what happens, or when they roll the dice. Instead of a binary pass/fail state, the game uses a good result/good result with complication/bad result, interested in consequences as opposed to success and failure. When the GM speaks, nothing never happens!
The rules are divided into smaller chunks, called moves. A move consists of a trigger, which is usually something a character does, or an event that occurs, then a mechanic for deciding the consequences, often rolling 2D6 and adding a stat, and the consequences, which the move will direct you to something interesting happening. Character sheets have everything you need on them to create and play your character. Character creation is fast, consisting of a few decisions that help describe your character, their motivations, and their origin You also have bonds with the other players' characters. This gets the players talking about their characters' relationships with each other very early on in a way that shapes the kinds of adventures they will have.
The GM has rules as well, helping them decide what to say in any given situation. The game plays very fast, bouncing between the GM and players and the rules very smoothly. Everyone describes what is happening, or what their characters are doing, while paying attention to see if they trigger a move. When a move is triggered, it steps in, directs the action to a particular consequence, and steps back out.
The game doesn't have a setting of it's own, but instead encourages the GM to ask questions in the setup phase to establish the setting. The benefits of this method is that there's no great dump of information that the players have to wade through. It also allows the players to introduce elements they are interested in and shape the types of adventures they will engage in. As the game goes on, and more questions are asked and answered, the world fills up with exciting and interesting details.
Dungeon World can be a paradigm shift in focus for people used to more traditional RPGs, with it's emphasis on improvisation. Players and the GM will need to rely heavily on their imaginations to simulate the world and bring the consequences of their actions to life. Tactical depth in the game also doesn't come directly from master of the rules, but of positioning your character in the events and places that allows them to get what they want. This makes talking about what's going on, and the GM being honest about the situation and consequences very important, so the players can make informed decisions.
If you are looking for something that plays fast, and requires very little preparation time to set up, and you love making up adventures as you play and bring surprised, I recommend Dungeon World wholeheartedly.