The players are exploring a new wilderness (perhaps not unlike this one or this one), and they discover a ruined temple. They remember that that crazy old trader back in the Imperial Town said a Black Dragon nested somewhere in here. They beat a hasty retreat; they know that they'll need backup or better specialized weaponry before going toe-to-toe for the dragon's horde. They sneak off to find easier pickings elsewhere.
Only a few miles away, on the remnants of a long-forgotten trade road, they are accosted by a band of hobgoblin bandits. It's a tough fight, but they manage to eke it out. Under some less-than-friendly persuasion, the one hobgoblin they capture tells them that there is an entire horde of hobgoblins in the foothills to the nearby north.
Moving on, they come to a small village on the edge of the river. After engaging in some bartering, they learn that the swamplands to the south are relatively safe, with a very few lizardmen and the sunken remains of an old shrine. One villager tells a crazy story about this abandoned dwarven mine down the river--abandoned except for an Ogre warlord and his followers, that is. But supposedly, there's still some mithril left--how much, no one can say. It is said the warlord controls at least one of the passes through the nearby foothills, though--and can be bargained with.
In the preceding example, level-appropriate challenges do not instantly appear. There are threats in the various hexes of the above adventure map that are fixed. It is up to the players, through exploration, trial and error, negotiation, information gathering, and interrogation, to gain an impression of the threats and features of their adventuring area. Challenges in an area do not scale because of their level; the world does not change itself because of the level of the players.
This can be included as an element of "sandbox" play, but I also see it as setting "organic" boundaries. There is no artificial challenge level adjustment--it is up to the player's characters to decide when and where there challenges occur. They decide the path of least (or greatest) resistance through the world, as well as when and where to gamble when the reward might be worth it. This approach puts info gathering and character interaction with the world at a premium. At worst, if the characters are reckless, heedless, or supremely unlucky, it can result in more Total Party Kills. At best, it makes the game less predictable, increases the feeling of exploration and danger, and gives the players a sense that the world will not bend to their whims or forgive their weakness.
I doubt I'm doing anything more here than codifying for my own thought process a practice that gamers have been using for a long time, but I do believe I prefer it to the "challenges remain or adjust appropriate to level approach".
Anyone use this style of encounter planning? Anyone shy away from it or dislike it? Thoughts?