Pathfinder - Commoner, Expert, Aristocrat
DnD 3.0/3.5 and Pathfinder included rules for several NPC classes: Adept, Aristocrat, Commoner, Expert, and Warrior. While the Adept and Warrior are essentially less-powerful versions of heroic classes the Aristocrat, Commoner, and Expert make up the vast majority of the faceless NPCs who populate the game world. The existence of these classes has some very interesting implications for the economy of a fantasy world.
The Aristocrat is the NPC class for the 1%. The privileged, wealthy few who are either born into money or acquired enough of it to make it into the ranks of the world's nobility. Aristocrats might make up many of the important NPCs in your game world: members of government, wealthy benefactors, diplomats, and merchant nobility can all be represented by the Aristocrat.
Aristocrats have had an important role throughout much of human history. Human societies have historically had a wealthy ruling class which wrote and enforced the rules of society. While this has historically involved varying levels of brutality, corruption, and general awfulness, it has been an important part of society. Commoners depend on nobility for protection, organization, and employment as much as nobility depend on commoners for labor, soldiers, and food.
Some Aristocrats might be particularly skilled or famous commoners who made a name for themselves and live comfortably by riding on the coat-tails or traditional nobles. These lucky few might be famous smiths or scholars, or they might be nobles who chose a tolerable profession instead of managing their family estate. Historical examples include famous artists like Michaelangelo, and literary examples include magical artisans like Celebrimbor, the elven lord who forged The One Ring.
Aristocrats get 4+ skill ranks, and have a considerably larger list of class skills than Commoners. Many Aristocrats will likely have ranks in social skills like Diplomacy, and many might completely forgo ranks in Profession, choosing instead to live on their family's inherited wealth or on the taxes of local peasantry in place of an actual profession.
When looking at an NPC, it is generally safe to assume that they are a commoner until the GM provides enough evidence to prove otherwise. Commoners are far and away the most common class in a Tolkien-esque fantasy world, and this needs to be the case for the world to make sense. Commoners are the manual labor of the world: they work the farms, transport goods, and make up nearly all of the hard labor in the world.
The existence of commoners is historically consistent. The vast majority of laborers throughout human history have been unskilled, and one average laborer was largely interchangeable with another average laborer. Unskilled labor is common, cheap, and effective with a little bit of coordination.
In game terms, the Commoner gets 2+ skill ranks, and has access to a small set of skills including Craft and Profession. Most commoners will probably be level 1, but experienced NPCs might have a few class levels. Assuming that NPCs max out their Profession skill, they will likely have one career in which they are invested. This flies in the face of what a Commoner should be, but it is an unfortunate side effect of the rules. To make this make sense, limit Commoners to unskilled professions like Builder, Miner, Porter, or Teamster. Things that generally involve a lot of tedious manual labor, but no education or training.
Often overlooked, the Expert is the most complex of the NPC classes, and has the most jarring implications for a fantasy world. Experts make up the worlds "middle class": skilled, trained labor in professions which require special training. The existence or absence of a middle class has deep and complex implications for your game world.
Historically, the possession of land has defined wealth. If you owned the land on which you lived, you could then rent that land to commoners who would then pay you to live there, often in the form of taxes. Trained professions allowed the middle class to accumulate wealth without the need to own land, and often without the need to own the often expensive tools of their trade. Trade guilds made this even easier, as they accumulated shared knowledge and shared the resources of their trades for mutual profit.
In game terms, Experts get 6+ skills and can choose any 10 skills as class skills. This makes the Expert extremely versatile, and Experts can make up any trained profession. Unlike commoners, it makes sense for Experts to take ranks in trained professions like Barister, Mason, or Scribe.
The existence of a middle class can have a huge impact on the feel of your world. The availability of skilled stonemasons can mean stronger walls, better roads, and more cities built primarily of stone and brick. The printing press, perhaps the most important invention to the middle class, means a wider availability of books and a more educated populace. Other professions can introduce architectural wonders like running water, sewer systems, and aqueducts. With a sufficiently stable economy, sufficiently large cities might include luxuries like parks, public art, and bath houses.
The lack of a distinct middle class typically means that the world feels more impoverished. Outside of the estates of the wealthy, structures are primarily made of cheap locally available materials (wood, mud/adobe, etc.), and luxuries are expensive and rarely available. Major economic transactions are almost exclusively made by nobility, which means that adventurers of any significant level will likely depend on noble patrons to employ them.
When building your world, consider the place of Experts in your world. Do they exist at all? Are they a new emerging economic class? Is the middle class an established part of the world's economy? How does the middle class interact with the nobility?