At first glance, the world looks rather similar to other D&Desque fantasy settings. There are various nonhuman races, various types of monsters, and, of course, there is magic.
However, magical energy has a source: The molten core of the world, from which it gradually travels outward throughout the layers of rock until it dissipates into the sky. At the surface, only fairly weak magics can be cast without constraints, and you have to go to the deepest depths of the Underdark to be freely able to cast the mightiest of spells.
D&D: You can only cast cantrips and 1st level spells at the surface. If you are underground, you can cast 2nd level spells, and deeper levels allow you to cast higher-level spells until you reach 9th level spells just before you reach the magma.
The ambient magic in the depths warps its inhabitants, which means that the deeper you go, the more bizarre and powerful creatures will you find, which sometimes survive by devouring magic alone. However, the ambient magic also anneals in in harvest-able form - strange fungal growths, crystals, even internal organs of creatures. These can be used to cast more powerful spells at the surface, though they are consumed during the casting.
GURPS: Ingredients from the normal-mana regions reduce the spellcasting penalty for the surface low-mana zone to -4. Ingredients from the high mana regions reduce it to -2. Ingredients from the very high mana regions waive the penalty entirely.
D&D: Ingredients from a region where you can cast a particular spell level freely also allow you to cast the same spell levels at the higher layers, including the surface - for instance, if you are in a region where you can cast 5th level spells, then ingredients from that region will allow you to cast the same spells in less magic-rich regions, including the surface.
(Note: Volcanoes are generally more "magic rich" than the surrounding areas since they present a direct access to the magma. However, since the mana dissipates directly into the sky, volcanic regions do not provide especially magic-rich ingredients.)
So, what does this mean for the campaign? Well, for starters most mages will generally lair underground. But it also provides a clear impetus for surface civilizations to venture into the depths - ranging from small-scale expeditions to establishing long, fortified trade routes and even entire cities. It will also encourage trade with the natives (just what are you willing to trade to the drow in exchange for their magic crystals?) - and if that's not possible, warfare.
For the player characters, it also encourages going deeper into the bowels of the world as the campaign progresses, as both the challenges and the rewards will increase further down. More than just a place to do the occasional quest, the Underdark becomes the focus of the campaign. As they increase in power, they can explore ever-stranger civilizations and locations until they reach the deepest depths.
Note: This was inspired by this RPGNet thread.