meat fighter badass solves problems by looking tough, being tough and having a badass follower that looks like a hyena
In my head there is this Platonic ideal D&D wherein players control characters and the characters all go off and explore and have adventures in strange and dangerous places. There are obstacles and players use resources (what's on the character sheet) and skill (cleverness, co-ordination, knowledge of the game, etc) to overcome the obstacles. It is fun in the way that games that are about resolving challenges and exploration are fun.
Character knowledge is one of those fun black boxes whereby player and referee get to peel back the protective flaps covering the games auditory pits and gently whisper questions into the game or at one another and then get something weird and interesting back. This character was a mucker of Nuln before donning the wizarding tunic they now wear. What might a mucker know about a particular sort of mud? The arid plains of Nuln are known for their mushrooms and nuts, does that mean the mucker knows a bit about what's good to eat in this forest?
The OD&D classes represent radically different approaches to character knowledge and problem solving. (this post here at 9 and 30 kingdoms is related to this)
Clerics, like Fighters, solve problems in a similar sort of sphere, both types of problems being of a decidedly fleshy nature. Magic-Users can, theoretically solve any type of problem at all but they can only solve so many in "an adventure" (which I take to mean in a single session, but later D&D refines to once-a-game-day-so-long-as-you're-getting-in-a-good-rest) and they have to figure out how to solve that problem in their weird wizardy way or they have to steal that knowledge from someone else.
Fighters exist as meat. Their plans are meaty, their actions take place in the usual meat space. They get ignored a lot when people talk about classes and games because they are generally so unproblematic/understandable. "Can my Fighter do this?" can nearly always be resolved with, "if you had the stats your Fighter has, do you think you could do it?"
Clerics are hopeful meat. They hope their god(s) pay attention and help out. It is generally a fun rule to treat the Cleric's spell list as mostly a description of the outer boundaries of what the god(s) are interested in doing for the Cleric this session. The Cleric prays over some of the Fighter's mangled meat body and hopes that things turn out well. Maybe the god is a Troll and the ruined limb heals itself over the next few minutes; maybe the god is an Ent and the replacement parts are some kind of muscle-wood hybrid because Ents don't totally "get" human parts; maybe the Cleric spent last night getting drunk (you know, carousing tables) and the Cleric's judgey white male god decides that while the Fighter is healed, the Fighter's wound is passive-aggressively transposed to the Cleric.
A Cleric doesn't necessarily know anything new or special about the game world unless it's revealed to them. It's like they have these giant monsters riding on their backs. The monsters help them sometimes and sometimes they don't and they impose rules on when they help and the general idea is that really, the real stuff is going on at monster-view level and the monsters are calling the shots and you're just a really advanced horse.
The life of a Cleric is someone's nightmare about religion.
Magic-Users, on the other hand, know special stuff. Maybe what they know is monster-view stuff which is why it's always so non-linear and knowing it is a vertiginous experience and why Clerics and Magic-Users aren't supposed to get along. Rote Vance (and pretty much rote D&D) is super boring here in practice, but the fundamental theory is interesting. Magic-Users traditionally get their magic by (a) being a little magical themselves and (b) cramming magical, weird, non-linear magic (which might be a monster or something like an invisible monster, maybe made out of dark matter, maybe an angel or aether a la John Dee) into what is probably some special lobe of the brain. (b) nearly always involves reading and being able to cast a spell is usually stated as knowing the spell (and/or having it memorized with the distinction being: can you cast this spell at all [know]? versus can you cast this spell now? [memorized]) and, since the start of the hobby, Intelligence is the core stat for the Magic-User (in OD&D INT also influences how many languages a character may know and in Greyhawk, even the number of spells they may know). All of which is to say that Magic-Users are and always have been about having that certain special information that turns what was an obstacle into an obstacle no longer. (Note also that Clerics in Greyhawk don't get extra spells for high INT as, Gygax notes, clerical magic is "given).
The whole spell-hunt meta game for Magic-Users is similarly about seeking out the knowledge you might want in the future to solve some other kind of problem.
This is kind of related to other posts (esp stuff on Magic-Users and Clerics and Deities) but mostly to stuff I'm working on.
attribution: Pieter Hugo