The theory behind this idea is explained in this post, quite broadly, and examined in this thread in a little more detail. The tl;dr version is this: 3e and 4e are designed with the encounter as the central unit of play. This paradigm changes that, moving it to the adventure in general.
Of course, changing the central unit of play is no small task, but here is what we are looking for: The Adventure is where the the character uses their resources in order to achieve success, and what limits a character's resources.
For that major change, the first step is on the DM's side.
An adventure, in mechanical terms, must hit the following notes:
- It must risk something, posing a danger to the party.
- It must have a distinct goal, something that it is possible to succeed or fail at.
- It must provide the following four challenge types: Combat, Interaction, Exploration, and Discovery
- Each adventure provides characters with all the appropriate treasure and XP to get them to the next level. Individual encounters do not award XP or treasure, only adventures.
The four challenge types are discussed a bit here, but the basic thrust is that, judging from the skills that a 4e character has, these are the four main things that any character engages in, in a typical D&D adventure.
In order to provide enough treasure and XP, you need to assign a full level's worth of encounter resources to your adventure. This, effectively, means 10 encounters, 10 Major Quests, 10 Minor Quests per character (usually, 50 Minor Quests), or some mixture of the three. In an adventure design, these are all headed under one umbrella: Tasks.
Tasks are the obstacles and goals that will lead to the completion of the adventure. They can be combats, quests, skill challenges, skill checks, rooms to visit, or things the party needs to accomplish. Major Tasks involve the whole party to accomplish some goal (skill challenges, combats, etc.). Minor Tasks involve only one character, and usually no more than 1-3 die rolls (skill checks, quick attack rolls, minion skirmishes, etc.). Each Task should have a keyword related to one of the four challenge types (Combat, Interaction, Exploration, or Discovery).
You can add tasks that you don't expect to be completed (Sidequests), or tasks that are open-ended, without an obvious solution, as well (the players can work out how to handle the solution for themselves). Tasks can be avoided or inessential. What is important is that the characters complete the goal of the adventure. Tasks are just a way to measure their progress to that goal in a concrete way.
You can just let those tasks sit there, waiting for the players to accomplish them, or you can present it in a more rigorously narrative fashion, with a reasonably even distribution in each session you will play (generally, 3 encounters, 3 major quests, 3 minor quests per character (~15), or some mixture of the three).
Any failed Major Task results in a failure for the adventure. A failure at a Minor Task simply means that the character that attempted it cannot attempt it — other characters may still try. If every character fails a Minor Task, it is a failure for the adventure.
Sidequests that are failed are simply closed to the party, and add a Major Task to the tasks necessary.
While normally a failed task results in a failed adventure (a party killed by goblins is pretty much dead), this doesn't have to be the case. A DM can implement setbacks instead. A failed task earns a Setback, and the adventure isn't failed until the party earns its third Setback.
When a setback happens, DM adds a task, and puts the characters in the midst of it. A party killed by goblins, for instance, may instead be held hostage, with the DM adding a Major Quest of "escaping the goblin prison." If the goblins kill the party 3 times, though, the party is finally killed, and the adventure is failed.
Earning Your Rest
In this design scheme, a character cannot simply take a short or extended rest. Rests are things the DM hands out, not things the players can assume to have.
A party can take a Short Rest, but if they do, the DM adds a Major Task (an encounter, a major quest, or a minor quest for each character) to the dungeon, as the inhabitants gather reinforcements, repair traps, and put the area on alert.
Completing a Major Task, or one Minor Task each, allows each party member to take a short rest "for free," without triggering an additional Major Task. This represents that the party has cleared an area, solved a part of the mystery, or otherwise has made enough progress to take it easy for a few minutes and pay attention to recharging abilities and healing wounds.
The party cannot take an Extended Rest while engaged in the adventure. They can, of course, sleep, keep watch, etc., but they gain no benefit from it during the Adventure. If the party needs to take an Extended Rest, they must quit the adventure, which means that they fail, and the ramifications of the
Completing two Major Tasks earns the party a milestone. At each milestone, each party member can take an extended rest "for free," without failing the adventure. They also gain an Adventure Point. This represents that the party has gone a significant way toward solving the adventure, and have managed to get some worthwhile rest.
Instead of Action Points, characters gain Adventure Points.
Adventure Points can be spent like Action Points, giving the character an extra standard action, but they can also be used to complete Tasks. An Adventure Point can give a character a re-roll on a Minor Task or a Major Task. Adventure Points can also be spent on certain rituals and powers that help in an adventure.
In a player-driven game, players can select tasks for themselves, that the DM then assigns an XP value to. For instance, if a character wants to become lord of the land, the DM can say, "That's worth five Adventures itself," and then position five adventures in front of the PC that will result, in the end, with them becoming lord of the land (5 levels later). A smaller goal, such as "Kill the orc that slaughtered my family" might only be a single adventure. "Make a good impression on the Duke" might be a Major Task (as a skill challenge involving the whole party), or even a Minor Task (a few skill checks involving mostly just that character). You might have a high quantity of un-formed tasks that are theoretically out there, waiting for characters to discover them. PC's may pick up and change adventures regularly, or even work with several adventures in tandem.
In a story-driven game, the DM can set tasks before the party. For instance, if a tornado is about to hit the city, an Adventure might be to survive the tornado. You might only have a few narrowly focused tasks that pertain to the timeline you have in mind — required tasks. The players can accept other tasks, and can come up with their own solutions for your tasks, but your tasks will happen, and must be dealt with in some way.
Your game may use a mixture of these styles, depending on how self-motivated your players are, or how assertive you want to be as a DM.