Yes, the title of this post is a bit hyperbolic. I don't mean to imply that the rules governing actions such as fighting and movement should be ignored for everything and everyone except the PCs. I refer more to the rules governing character creation and advancement. Apparently for a while, ideas of character classes and monsters and such have been percolating in my head, and this post represents a few of the crystals precipitated from that supersaturated solution.
Character classes have certain abilities, which are standardized by class and carefully (or not so carefully) scaled by level of experience. This establishes predictable boundaries and expectations for players when choosing their characters' paths in a campaign, and a schedule for the further development of their abilities. As such it is a useful tool, but not one that I believe was meant to be applied universally to PC and NPC (and sometimes even monsters!) alike. Classes are no doubt useful templates for building NPCs in the campaign world, but if we bind ourselves to the notion that every fictitious person existing in the campaign world must be governed by those same rules, it leads to absurdities like a village locksmith having to be a high-level thief so he can have a good Open Locks skill, or any apothecary who isn't a charlatan necessarily being a name-level wizard because he can brew a few potions. Taken to extremes, it leads us to such lunacy as giving every person of every profession a class and level to define his abilities: a 4th level peasant or a 15th level blacksmith or an 8th level merchant.
There's no need for a rigid system of rules governing the abilities of NPCs, though. NPCs may, and should, have whatever abilities the referee thinks they should have according to their profession and background on an ad hoc basis. Abilities normally considered the province of a character class can be assigned to non-classed NPCs at whatever level of skill seems appropriate. There's no reason in the world why a 0-level man who's been practicing the trade of locksmith for twenty years shouldn't be able to open locks with a 95% chance of success, while never gaining any more hit points or combat proficiency nor any ability to move silently, climb walls, or any other thief abilities. Maybe you decide that the high priest of the most prevalent religion in your world has never adventured a day in his life, but attained his position by being the most unwaveringly pious, devout, and humble exemplar of the faith, and you're perfectly justified in giving him the ability to invoke miracles (i.e. high-level cleric spells) and to turn undead as a 20th-level cleric while having the hit points and combat ability of an ordinary man. A woodsman might have the tracking skill of a ranger, without ever learning any great skill as a warrior. An apothecary need not be a magic-user to brew efficacious elixirs and philtres, despite the skill being reserved to that class in the rule book.
Sometimes you might want to give NPCs, classed or un-classed, an ability unavailable to standard PCs of any class. A common farm-wife or the general of the king's army might have prophetic dreams. This doesn't imply a need for a Prophet or Oracle character class with a carefully-crafted list of powers and notes detailing at which levels they're gained. A minstrel might develop a supernatural knack for charming rodents and children with his music, but again, there need not be a Pied Piper class. A sailor or a retired adventurer with a bad knee could be renowned for his ability to predict the weather. A slow-witted village child might spontaneously cast fire spells without knowing how or why, and despite having neither training nor aptitude for the magic-user class. A particular thief might gain the ability to become ethereal at will. A fighter might secretly have an inborn ability to communicate with animals.
In the former case, player characters may develop those skills by being a member of the relevant class and accumulating levels of experience. Those NPCs may have a high degree of ability in one skill, because they are truly specialists, and adventurers are by necessity generalists to a great degree. The NPC locksmith doesn't divide his efforts between a handful of thief skills, plus fighting, mapping, and survival; his job is to understand and make locks, period. If a character wants to be a locksmith, he's not going to be an adventurer, and so is unsuitable as player character in a game of exploration and adventure.
In the latter case, powers and skills might be gained in any number of ways, from divine favor, family curses, flukes of fate or nature, or freak accidents that for one reason or another cannot be reliably repeated. In any case, the recipients are exceptional, and didn't choose their gifts, and neither can a player character (excepting perhaps with DM permission at character creation.)
Sometimes, especially in the case of adversaries and enemies, it's appropriate to build otherwise "human" NPCs as if they were monsters, giving them Hit Dice instead of levels, and abilities beyond normal human ken. A coven of hags, cultists of a Cthulhu-esque Outer Being, or a unique knight who serves as guardian of a magical locale might all defy the categorization of standard character classes.
Naturally, you don't want to go overboard. The most potent or uncanny abilities should be reserved for the most exceptional characters, but this ought to be independent of game mechanics like class and level. It's an art and not a science; a formula that's part common sense, part game master's artistic license; a nebulous alchemy that would completely lose its magic if subjected to strict codification in class-and-level rules.