In truth I really dislike the "world hangs upon the shoulders of the PCs" when there are so many more powerful magnates to sort it out...
I'm not sure if it became clear in my post, but my AD&D campaign was an attempt to have fun with TSR antics and use as many cliches as possible. Think about it: evil mage, world threatened, the PC group "our only hope." It was just a laugh, but everybody thought it was relaxing to play a mindless AD&D campaign along with more serious games of Harn.
Oh, I actually created the world as the PCs progressed. It all started in the small village of Wayward, when a group of lonely adventurers noticed a new star in the sky...
the world doesn't stop on vacation
In my HM campaign, the world evolves around the players even if they do nothing. If (or when, rather) there's, say, a crusade, the players had better not spend the next ten years breeding and watering the flowers if they're going to take part in it. But they can
do it if they want to, but then they miss all the adventures that they might have had along the road. Well, with the current PCs I'm sure a crusade would get much interest from the players.
I've noticed that if there are no adventurer hooks for the characters, the players start acting unscrupulously and try to "take over the world" (once again). It seems that improving your social status is the first thing that springs to the mind of an idle PC. Not many characters decide to, say, start a witch hunt or just explore the world. This leads to even more evil or "unscrupulously neutral" (as I often describe my groups) characters.
A friend of mine ran a free form campaign for a long time before he started working on an actual plot. Unfortunately, he put forth the plot just when we were starting to get settled and powerful enough to actually have an effect on the world. We weren't a bit interested in his adventuring hooks, which only meant us taking unnecessary risks and gaining little on our own quest of world domination (i.e. building a political and economical power of our own). This lead to ruthless railroading which only made us upset. His world was also quite a lot on the high fantasy side, which meant that railroading was very easy.
For example, one night, a group of daemons emerged from a interplanary portal in the lord's bedroom of our (no, mine!) castle in the middle of the night! After killing off the other-worldly invaders, we and our guests, families and servants were all drawn into another world, from where we had no choice but to escape. Unsurprisingly, only the PC group survived, the last NPC dying on the doorstep to freedom (it was a laugh).
A better example of my friend's campaign was during the free form phase of the campaign. At one point we learned that there was a valley where some power, that would make munchkins of all of us, could be found. This lead to a vicious hunt for any and all information on the valley and its location. During our investigations, we also killed everybody else aware of the valley and its power. Our group was selfish, unscrupulous and ruthless, as we didn't want anybody else to become munchkin. Thus, we spared no expense and did everything possible to find the valley. I don't think I've ever seen a group as motivated on their quest than the one on its quest for the Hidden Valley.
Our quest almost failed when we killed the only man who knew of the valley's exact location and found out that he had lied to us! Luckily we found his diary later on at a wayside inn (which, uh, caught fire later).
Many characters died on the quest, but ultimately we found the source of power.. The disappointment was great when all we found was a couple of rotten Earthmaster artifacts (haha). Oh, and we got a random psionic skill as well. Well, we had had a great quest, and I still think it was very much worth it. It was just that much fun. It did take the luster of playing unscrupulous characters for a long time, though: I don't think there is a sin that at least one of our characters did not commit at one time during our adventures. Well, at least we had a set goal, so it wasn't just "playing bad" that many evil groups do.
I think my point was that in free form campaigns, "good" groups tend to start following orders or looking for ways to serve (which forces you to generate adventures all the same), whereas "Evil" groups start looking for their own good, and thus create their own adventures, (see Patrick Nilsson's
post above) and often commit so many atrocities that it soon gets out of hand. Neutral groups tend to seek only monetary gain at first and later, when they're rich enough, turn to either end of the moral spectrum.