- Is the Cleric really an archetype? Is the Magic-User?
- Why isn't the Thief an archetype? How can we fix this?
- How do we take an iconic fictional character type and distill a class from it?
- What do we do about specialized versions of each class?
Thus, the Fighter isn't defined by "weapon skills"; it's a more general ability, including the ability to use any weapon, to fight many opponents, and pretty good saves so that the Fighter can face problems head-on. Similarly, the Magic-User isn't just a spell-caster, but can also make magic items and use those that cast spells (scrolls and wands.) The M-U isn't just the robed scholar of late fantasy literature, but any character who relies on magic to solve problems, from the heroes of fairy-tales who use magic gifts wisely, to legendary and mythical figures with supernatural powers. The literary or thematic archetype (function within the story) can change, so that we have schemers like Medea and Gwydion, wise men like Math ap Mathonwy, and figures like Merlin who change from version to version. The broader archetype (function within the encounter, or how the characters deal with stuff) is still "uses magic" for all.
Neither the Fighter nor the Magic-User is obligated to perform the actions associated with their class. They can outsmart opponents, bribe them, intimidate them, or flee; they can avoid one or all types of opponents, focusing instead on abandoned areas filled with traps and puzzles. They can climb, run, swim, jump, move objects from place to place, build stuff, take things apart. All the class says is "here's something special you can do, so you might want to think about this when trying to solve problems."
Now, you can list example characters for each class, to offer as suggestions for players. You can also list other characters who seem similar to you, to try to distill a new class from those examples. What I would say, however, is that you shouldn't base the class around what those characters did, but what choices the characters made. Hercules, Theseus, Perseus, and Beowulf are all theoretically Fighters, but we wouldn't want to list their class abilities as "Strangulation, Berserk Rage, Yarn Use, and Mirror Targeting". Those are elements that distinguish each character from the broader class.
The Cleric, as I noted, is a little bit of a problem. I think as the class has come to be understood, it is no longer archetypal, to agree with Stuart for a moment. The Cleric is more of a mix of Fighter and M-U, with some specialization, and represents something we have few literary examples for. But curiously, I think the origins of the Cleric as a Van Helsing stand-in points to the Cleric being more than a mere priest or crusader. There are actually a lot of characters whose first instincts are not to rely on physical might or magical knowledge, but on faith in a higher power, and who seem driven by devotion rather than bravery or esoteric lust. There's the stories about saints, not all of whom are missionaries or priests; there's fairy-tales involving characters being saved from supernatural danger by making the sign of the cross; there are pre-Christian equivalents to these, again not always actual priests, but just heroes noted for their piety. So, there's definitely an archetype there, it's just been tainted by non-archetypal thinking.
Similarly, there are definitely tricksters before The Hobbit, beginning with actual Trickster-Gods and proceeding through Odysseus, the Brave Little Tailor, Puss in Boots, and Jurgen. The problem is that not all of these are thieves; in fact, many of them aren't. And not all of them are "physical" tricksters (using disguises, traps, or physical props, like Odysseus.) Many of them rely on charm and clever words. I think FrDave had the right idea in his post on the Thief when he started focusing more on surprise and alertness. I kind of headed that direction with the Trickster redesign of the Thief, but I think now it should just go all the way: make the class all about surprise, giving the Trickster the uncanny ability to re-roll surprise and add a bonus to surprise or avoid surprise (alertness.) The mechanical abilities of the Thief get shunted into another class, the Tinkerer.
The Tinkerer represents another principle: if a class should not be about what a character can do, but what a character chooses to do, then conversely changing the focus of a class without changing the general structure shouldn't be a big deal. Technically, the archetype is the same for both the Tinkerer and the Trickster, as well as the Charmer, to a certain extent; they all are defined as adding a bonus to a specific action. This is what Flynn called the Expert, but there's a problem with Expert classes as typically presented: they are built around specific skills, instead of an uncanny ability with any skill they should happen to try.
It's not that you can't have a skill system (although I prefer a binary "yes, you know how/no, you don't" approach.) It's that skills should be independent of class, as should some specific abilities, like immunity to sleep or innate invisibility. These things distinguish two characters of the same class, not two classes. "Berserk Rage" might be a feature of a particular Fighter, but it works better if there isn't an entire Berserker class, just as the Fireball spell is something an individual Magic-User might learn and become known for, but there shouldn't be a Fireball Mage class.
The key to defining a class archetype, which I haven't been expressing very well, is this: find one or more characters that you think represents the archetype and look past what they do at their motives for doing things that way. Why did Bilbo use a ring of invisibility to sneak up and spy on a dragon, instead of rushing at the dragon with a sword? It's because he prefers stealth to direct action. Why did Van Helsing hunt Dracula? It's because he felt a moral and spiritual need to oppose the works of the Devil. Why did Dilvish usually fight opponents instead of using the extremely powerful, magical Awful Sayings? It's because, even though he knew magic, he was primarily not a Magic-User, but a Fighter; he felt a need to face foes directly.
In a way, I don't feel I've adequately covered the topic, but this post has gotten awfully long, so I'll give the topic a rest for now.