Since the players will need to pick their classes before play starts (imagine that!) giving a brief description of how each class (and race as class) fits within the world falls upon my shoulders. Not only is it the first bit of world building, but it lays the foundation on how the rest of the world works, as the concepts and tone of the classes reflects the overall concept and tone of the world. Let's take a look:
Clerics: Clerics are divine emissaries, sent to spread the word of their religion and smite undead where ever it may lie. Players creating clerics are free to invent their own gods and religions based around those deities. The gods and religion are very much a part of the world, however, the gods do not take the active role that many do in other fantasy settings. Much like real world religion, gods are not expected to perform miracles or send down angels to protect man. Because of this, not every person (no matter what race) is as religious as they are in other worlds. In fact, the idea that every cleric, no matter what god/religion he belongs to has the same abilities and spells like all the rest have some people speculate that a cleric's power doesn't come from the gods at all, but some far more sinister, dark, and secret power. Clerics are not always welcome in some places and the word of their god may invoke little fear and have little meaning in some civilizations.
Fighters: These guys are pretty classic; trained at various guilds and war colleges, fighters adventure for riches, fame, honor, or most likely a combination of all three. Fighters are the most tolerated of the "adventurer" profession by the common folk, especially when they are hired to save them from some rampaging menace. A fighter's ability with all manner or weapons and armor, combined with a healthy stamina and oft times macho attitude means that sometimes the fighters themselves can be the rampaging menace as they often look down on the common folk.
Magic User: Arcane magic is not nearly as trusted as divine magic is, mostly because of its potent offensive capabilities in comparison to the more benign (and more helpful to the "common folk") form of casting a cleric does. There are no great magic guilds or great wizard towers in the center of cities, instead, magic users are taught in secret, mostly in underground areas or far from civilization. Magic users are ambitious, inquisitive, and not content to live life without asking why. Many religions don't like a magic user's desire to learn the "deeper meaning" to everything and the more zealous man may burn a magic user at the stake lest he can prove that the gods don't exist.
Thieves (Delvers): There are thieves throughout the world, often in urban areas where prey is most plentiful. Many form guilds in order to regulate their "trade" and work together for bigger profit. These thieves are not the ones that are out adventuring however. In this world the "thieves" available as characters are known as delvers and they are the equivalent to a tomb raider like Indiana Jones or Lara Croft. They are the ones investigating old catacombs for ancient scraps of text and hidden treasure or running the goblins out of an old temple to preserve the artwork still left inside. Delvers share an inquisitive nature with magic users and the two often get along in the pursuit of similar hidden knowledge, atleast until both want to possess the same ancient manuscript.
The above are all human classes, of course, and humans are the de facto dominate race on this world. In the area in which the megadungeon is located the vast Valadian Empire holds sway of much of the land, claiming all they can see as belonging to the humans. However there are the race as classes available and each has a slightly different spin then their Tolkien inspired classic counterparts.
Dwarves: The dwarves are a dying race, many of their females are barren and unable to conceive and even among those that can, many children are stillborn. The once vast dwarven clanholds now lay mostly empty as the race dwindles in number. Ever stubborn, if the dwarves cannot pass on their legacy through offspring, they have chosen to do it by deed. Mix Klingon with your standard barbarian fantasy troupe and add a little dash of Warhammer Slayers and you have the dwarves of this world. They look to die gloriously in combat, preferably after saving nations or the entire world from unspeakable evils. Dwarves are well respected in the human communities and would be a welcome part of them, if the dwarves weren't so insistent on moving on to the next area of trouble, always looking for a glorious death.
Elves: Elves are alien and distant in comparison to humans. They are immortal and not born of this world, instead migrating from the land of the fae for reasons unknown to all but them. Humans to them are simply a curiosity, short lived, seemingly impatient, but possessive of fierce emotions that an elf could never replicate. To the humans, elves seem cold, distant, and often times uncaring. An elf can see a plan work itself out in the span of centuries instead of days, and see no issues with sacrificing the lives of short lived creatures (intelligent races included) to further these plans. They are master of both marital combat and arcane skill and are often called the Eldritch Fae. Elves most often adventure out of sheer boredom or while on the hunt for (by human standards are) ancient artifacts lost to time. While tolerated in human lands, both races tend to agree to disagree and not deal with one another if possible. The exception is other adventurers as they can prove useful to a elf's plans. Players wishing to play elves should really try to separate them from just "graceful humans" and play up the alien, detached nature.
Halflings: Halflings are born of human parents and are often seen as a curse laid down upon those that have sinned in some way. Halflings that survive to adulthood are stunted, thin, and often the host or one or more physical deformities, be it odd skin pigmentation, patchy hair on their bodies, a small vestigial tail, misshapen hands, etc. Most halflings are put out into the woods to die of exposure, though some parents take pity on them and raise them. Others have survived in the wilderness and now live on the outskirts of human settlements collecting those halflings left to die by their parents and raising them into adulthood. Halflings can never produce offspring of their own as they're born sterile. Halflings are rarely accepted in human society (outside of entertainers of the "freak show" variety or jesters for nobles) and many look to adventuring as a way to sustain themselves. Adventurers are already often seen as outcasts of society and many are much more accepting of halflings then the common masses.
Those will get cleaned up and transferred to the Portal in due time and provide players with both a first look at the world and hopefully some inspiration for character concepts.