Baiting the Hook

Examples of hooks that will bait a player into wanting to play your game:
Cultural: The world is based on a culture that is interesting and unusual. Examples include a game world with a Japanese flavor or a campaign set in ancient Greece. Several official D&D settings have chosen this approach, including Kara-Tur, Maztica, and Al Qadim.
Environment: An unusual environment dominates the game world. Imagine a campaign set entirely below ground or a game that takes place on a water world with a handful of tiny islands providing the only available land. Aris, the game world developed here previously, is based on this option -- it's entirely blanketed by forests.
Classes/Races: These campaigns limit all the player characters to a single class or race. Imagine, for instance, a campaign in which all the player characters are rogues or all the PCs are elves.
Opposition: Sometimes, basing the campaign around a particular monster or adversary can provide a workable hook. An effective game world might be completely dominated by undead or you might build an interesting world that is ruled by dragons.
Situation: Some effective hooks are simply the byproduct of an unusual situation that dominates the campaign setting. Imagine a game world that is on the brink of an impending apocalypse or a world in which the sun never rises. Here's a sixth type of hook to add to that list.
Inspiration: Adopting the works of a specific author or artist as the inspiration for your world can sometimes provide a workable hook. Examples might include a game that features the sort of whimsical fantasy often found in the films of Terry Gilliam (such as Time Bandits or the The Adventures of Baron Munchausen), or a campaign inspired by the florid fantasy of Lord Dunsany.

The New World

This time, "Dungeoncraft" is going to focus on a hook based on opposition. This new campaign setting will be dominated by dinosaurs. In fact it's a world in which there are almost no mammals, and various sorts of dinosaurs are used as mounts and beasts of burden. Imagine an enormous apatosaurus sporting war paint and carrying a battle platform on its back, or a knight mounted atop an ankylosaurus. In addition, wild and fearsome dinosaurs dominate the wilderness areas and prey upon any intruders who violate their territory. The aim here is to lend the world a "land that time forgot" atmosphere with primitive warriors wielding stone axes and struggling to survive amid harsh surroundings. Altogether, this new world will be less civilized than Aris, and its inhabitants will be a great deal more savage.
This path has several interesting implications, many of which will present some formidable design challenges.

Primitive Cultures

First and foremost, the cultures that inhabit this new world will be a great deal more primitive than those that dominate a typical D&D setting. One obvious implication of this is that there will be a much lower level of technology available than the D&D rules presume. This lost world has advanced to roughly the level of Earth's early Iron Age. While there are primitive metal swords and spears available, advanced metalworking (such as that necessary to create advanced armor types) should be almost nonexistent. This will change the choice of weapons available to the players, but more importantly, it might upset the game's balance. Restricting access to the more effective armors, for instance, might radically reduce the combat effectiveness of the average party, particularly against foes like dinosaurs.
Furthermore, the lack of advanced technologies might force us to reevaluate some of the D&D character classes. Without access to various weapons and armors, certain classes lose important capabilities and proficiencies that might throw them out of balance with the other classes. Most likely, it will be necessary to either restrict the available character classes or grant some classes new abilities or proficiencies to make up for these liabilities. Similarly, the primitive nature of the cultures that dominate this new world will force us to reconsider several of the D&D character races. The idea of coarse, unrefined cultures clearly contradicts the vision of elves presented in D&D, for instance. Again, it might be necessary to either limit the selection of available races, slightly rethink some of the standard races, or evolve all new races.

Magic

If you play "by the book," D&D's magic system isn't intended for this sort of primitive environment. The traditional D&D setting is dependent upon the classic notion of a wizard's workshop that is stocked with expensive laboratory equipment and dusty tomes full of arcane knowledge. Spellbooks, scrolls, and magical research all play prominent roles in the traditional setting. On this lost world, it's difficult to imagine encountering these things very often. Such primitive cultures certainly wouldn't develop many written materials of any kind, precluding an abundance of spellbooks and scrolls. Similarly, conditions are so harsh that it is difficult to imagine any but the very richest inhabitants of the planet amassing the wealth necessary to construct even a modest wizard's workshop.
Perhaps the easiest way to address these problems is to exclude magic (in the form of wizards) from the campaign altogether or to make magic-using characters incredibly rare. Although both are perfectly valid approaches, it's usually not a good idea to alter the basic essence of the game so radically. Spellcasters and magic items give D&D a great deal of its unique flavor and play an important role in balancing the game. Removing them certainly changes the tenor of play, and it might also severely weaken the players' adventuring party, forcing the DM to take other steps to correct the problem as play progresses. For these reasons, we're going to allow spellcasters and retune the magic systems to tailor them to the new world.

Harsh Reality

As hinted, this sort of "land that time forgot" atmosphere implies a harsh world in which mere survival is difficult. Although it's already decided that the humans and other intelligent inhabitants of the world have managed to tame some of the dinosaurs, the largest and most fearsome of the creatures are still the undisputed masters of their environment. This dominance is reinforced by the humans' lack of advanced technologies and sophisticated armors. Just to clarify the intent here, we don't imagine the adventures set on this new world to be any more difficult than the standard D&D adventure. Although the going is tough, the average player character should be as durable as ever. Rather, it's civilization as a whole that struggles. The life expectancy of a common inhabitant might be as low as thirty-five. The average human tribe spends so much time fleeing reptilian predators and natural disasters (such as volcanic eruptions) that there is little time to lay down any roots. As a consequence, food is probably somewhat scarce, and many of the tools that adventurers from other worlds take for granted are relatively rare on this world.
At the same time, let's not forget that dinosaurs are formidable opponents in D&D terms. Take a look at their statistics in the new Monster Manual. Even a relatively small carnivorous dinosaur is more than a match for a low-level player character. If dinosaurs are common on the new world, such encounters won't be uncommon.
For these reasons, it's probably important to beef up the effectiveness of the typical beginning adventurer relative to the rest of the world's inhabitants. We'll want to make sure that the average adventuring party is capable of standing up to the occasional dinosaur and surviving the rigors that limit the population's life expectancy.

The Past

Finally, the lost world approach has one other interesting implication. A compelling and vibrant past is usually a critical component of a successful D&D game world -- it's the past that propels most D&D adventures. Aren't all those dungeons usually the remnants of long-dead ancient civilizations and forgotten cults? Aren't most important magic items the subject of legends that date back several centuries?
In a lost world situation, it might be difficult for the players to easily grasp the campaign's past. If the cultures that presently inhabit the world are so primitive, what could have preceded them? Cultures that were even more primitive and even less refined? How could cultures like that leave any sort of legacy or lasting ruins in their wakes? In order to make it easy to create adventures set on this new world, we'll have to pay special attention to these questions as the campaign develops and make sure to create a past capable of capturing the players' imaginations.
This brings us to the Second Rule of Dungeoncraft. Now that an important detail about the game world has been created (the hook), we need to create an appropriate secret that is tied to that detail. An effective secret will not only provide me with an interesting springboard for future adventures, but also will go a long way toward solving the problem with the past as well. When searching for an appropriate secret, we made a list of the fundamental characteristics of dinosaurs, looking for possibilities. An obvious one immediately leapt out: Dinosaurs are extinct. Thus, the dinosaurs on this new world are on the brink of extinction themselves. As play begins, the world is undergoing a formative change. Unknown to most of its inhabitants, control of the planet is slowly but surely passing from the ancient dinosaurs over to the scrappy human upstarts. Further, the dinosaurs are somehow connected to an ancient and long-dead civilization of intelligent reptiles (lizardfolk in D&D terms). Somehow, long ago, the sorcerers of this once great civilization made a terrible mistake and unleashed some kind of terrible disaster that wiped their people from the planet and is still slowly killing off the last of the dinosaurs now. Thus, the world is probably dotted with the remnants of the lizardfolk civilization, making for some interesting adventure opportunities. As we continue to add details to the world, we'll have an opportunity to flesh out the nature of the ancient catastrophe and decide what it might mean for the modern adventurers.