Dungeons and Dragons 4th Edition is, by default, embedded in a campaign setting known as "Points of Light".
Points of Light is a significant departure from previous campaign worlds. It does not offer a fully-realised geopolitical landscape in the style of Dragonlance or the Forgotten Realms. In instead offers an unmapped wilderness into which individual adventure locations can be freely inserted.
There's several reasons for the change. One is that larger published campaign worlds come with baggage. In a setting like Dragonlance or the Realms, it's easy for player characters to be overshadowed by high level NPCs and have their stories pale beneath sweeping global events and epic international conflicts. Players can come to the table with unhelpful pre-conceptions based on earlier experiences with the world. DMs can feel pressured to absorb fantastic amounts of information in order to present a canonical "real" version of the setting to their players.
More importantly, though, the change highlights the core assumptions underlying D&D. The D&D universe is one where civilisation is scattered and fragile. Small "Points of Light" are divided by miles of wilderness teeming with lawlessness and horror. It's a world in which armies and constabularies are either absent entirely or deeply unable to clear back the darkness.
Points of Light is a world that needs heroes. It has problems that can only be solved with swords and spells, and those problems are so numerous that a nation of heroes could strive for a hundred lifetimes without stamping out the last of the evil. The gods are not active, the kings are not brave, and such elder wizards as exist are provincial and un-charitable. There are no higher forces making the players redundant - their characters are literally the last line of defence.
You can see the problem of canonical settings in Marvel or DC comics. Before any new hero can get a shot at defeating Dr Doom's evil plans, the writer first has to establish that the Avengers are off-world, the X-Men are busy, and S.H.I.E.L.D. have their hands full. Before an up-and-comer can go toe-to-toe with Darkseid, we need to know why the Justice League, Justice Society, Green Lantern Corps, Teen Titans, Outsiders, Doom Patrol and, in a pinch, Batman, have not already dealt with the problem. It's no fun feeling like the night-shift janitor who only gets called in when all the competent people want the day off.
Points of Light is modular. If you have five adventures describing aspects of the setting, you can use three and ignore two without leaving suspicious blank spots on the world map. There's no global cartography and the relationship of one location to another need never be better defined than "to the north and east". Moreover, if you pick up a Forgotten Realms product and like the sound of Shadowdale, you can insert it wholesale into Points of Light without worrying about its larger context.
This will all sound familiar to the experienced player. It's the way many of us have built up D&D campaign worlds since the earliest editions. Points of Light gives official sanction to this kind of simple patchwork design, and directly offers it as a design model to the beginning DMs who will get the most use out of it.
The theme of Points of Light is something that Thunderspire Labyrinth takes to heart. Thunderspire's quest hub is the Seven-Pillared Hall, a small community of traders presided over by the mysterious Mages of Saruun. The Hall lies within the underground ruins of the long-abandoned minotaur city of Saruun Khel, better known today as the Thunderspire Labyrinth. While the Hall may be a safe haven, the remainder of the Labyrinth is haunted by lawless humanoids and horrific below-ground predators.
The Hall itself plays home not only to humans, halflings, and the usual array of civilised races, but also to drow and duergar, emissaries of the malevolent Underdark races sent to the hall on semi-peaceful missions of trade. The Seven-Pillared Hall is a genuine frontier town, eking out a profitable existence in a place where an organised community should not be viable.
 Points of Light may be well-designed for encouraging beginning players, but it's far less useful for selling a series of novels, something that has traditionally been a profitable sideline for Wizards of the Coast. This approach is reflected in the D&D product line as well, with a much greater emphasis on core rulebooks and far fewer setting-specific products. What has changed in WoTC's market position to prompt this shift away from setting-based merchandising?