This is an attempt at an answer. It's only about half-coherent, and I can't tell whether it's obvious or just obviously wrong. I don't love it, and I don't expect you to, but I figured I'd get a rough draft out. I guess my new bar for 2018 is a post a month; maintenance doses.
D&D is an artifice for character-building for the participants (traditionally, poorly-socialized young men). Success in OSR D&D requires a number of properties which are useful in real life, which may be worth taking the time and energy to develop (and to encourage one's acquaintances to develop) in low-risk environments.
What are the lessons that various antiquated features of OSR D&D are aimed at teaching?
- Stats in order - Play the hand you're dealt. As Hamming said, "I will do the best I can with what I got."
- Prime reqs - In terms of class selection, you can fight your rolled "nature", but you'll get a lot further faster if you play to your high stats. Different classes demand different virtues of their players; thieves need risk-tolerance and faith in the rest of the party to come rescue them, wizards need patience, caution, and careful spell-timing, fighters need courage, persistence, and willingness to sacrifice themselves, and clerics need humility, the ability to accept that sometimes you're not the star of the show. Prime reqs push you towards classes that you don't usually play, and situations that test virtues you might not be so good at.
- Mixed-level parties, characters with very different stat distributions - Life isn't fair. If you're on the weak end, resenting the strong won't get you very far; work with them for mutual gain. If you're on the strong end, treat those weaker than you well, because you might have a reversal of fortune at any time and end up back at the bottom of the heap.
- Monoclassing, minimal build - You are not special by default; you are Joe Fighter by default. If you want to be somebody that people will tell stories about years later, it's on you to do something notable, not just to be something notable.
- Mapping, encumbrance, rations, Vancian magic - Come prepared, plan ahead, pay attention to the details. Neglecting them can get you killed.
- PC death - Memento mori. Some day, you will die. Yes you, reader. You can run from it, or you can accept it, figure out what you want, and go get it or die trying. It's important, I think, that unlike eg Call of Cthulhu or Paranoia, death is not short-term inevitable in OSR D&D. You need players to get attached to their characters, and that doesn't happen when death is too frequent.
- TPK / Near-TPK situations - How panic-prone are you? If other players are panicking, do you keep your head, or do you catch the panic? Can you calm others? Watching panic spread through a group of players is really remarkable. When you get crushed, do you ragequit, or reflect calmly on your mistakes and return for vengeance? The reason PCs don't make morale rolls is that their players are dealing with their own, very real, morale.
- Levels / fully-quantitative and exponentially-scaling advancement - If you want to get ahead, you have to be willing to take some risks. After the first couple levels, it doesn't just happen anymore with any sort of regularity. You need to say "I want to level", to choose it. And then you need to put in the work, take the risks, and probably get a little lucky. And that's true of life too. I'm hitting this point with my career, where I've done well but the organization I'm at is just small. I find myself with a choice between slow, pretty-safe advancement over the course of years, or choosing to prioritize advancement and doing the work to make that happen, either somewhere else or via personal projects in my free time. Since I'm here writing blog posts, you can guess how well that's going.
- Phases of play (dungeon / wilderness / domain) - Advancement has side effects, most of which involve new challenges, more responsibility, more patience, and more paperwork. This is what happens to conventionally-successful people. You can't have your cake and eat it too; you can stay in the dungeon forever, but you're not going to get over 6th level if you do.
- Old-School Wish - Be careful what you wish for. Someone was once very surprised when I mentioned that I give out wishes in my games; she said "But wishes destroy campaigns!" And I laughed. No, wishes are temptation.
- Rulings, not rules - Negotiate for things that you want. Don't appeal to authority; convince me. Whine, bullshit, do math, resort to bribery, whatever, these are valid approaches and you should learn them all.
In conclusion: I really do think OSR D&D presents its players with challenges which are well-suited to the development of mindset / personality traits which are adaptive, and rewards them for performing those virtues, not merely playing someone with those virtues. Other games foster different virtues; 3.x rewards you for reading rulebooks, finding loopholes, and doing math, which are very useful skills. Traveller... rewards you for automating the trade system, which is something, I guess.
What place does Fun have in this conception of D&D? I agree with Tao that Fun Is Not The Point. Fun is, however, a necessity. If your game is not fun, your players will leave before learning the lessons the game could teach them. So strive to make your games Fun Enough (probably Type 2 Fun); there's a balance. Unfortunately there may be a race to the bottom with fun; games which are more-fun tend to outcompete games with are less-fun in the marketplace for players. These games which are more-fun also tend to lack the features which promote controlled but real adversity, and consequently growth. I do not know how to resolve this problem yet.
What place does Narrative have in this conception of D&D? Likewise, narrative is a device. Humans like it, and therefore you can exploit their preference for it to make your game more appealing, and consequently more effective at cultivating virtue. I have been wrong about narrative. There might even be some aspirational "play the sort of person you'd like to be" upside to dealing with characters and stories, but that's not something I would know anything about. Classes are a very Jungian structure to begin with...
What place does Simulation have in this conception of D&D? If you seek to prepare your players for Real Life, making your game reasonably realistic (at least to the level of "actions have consequences, which you can predict by analogy with real life, except when noted by the rulebook") is sensible. Simulated details provide a hook for Attention to Detail, and can also factor into Convince Me. I have probably over-invested in simulation in the past (but it was fun for me, so not a total waste I guess).
What place does Game have in this conception of D&D? The game element, of luck and challenge and risk and reward, is pretty central. But I suspect one of the lessons to be learned from OSR D&D is that yes, you can play the game well, and you can win it, but it's sort of hollow and the reward of power is tempered with paperwork, as opposed to setting out to do your own thing and winning on your own terms, like Rary.
What is the role of the DM in this conception of D&D? Courtney hit the nail on the head with "shaman leading the group's collective vision-quest." You are here to help them become what they could be; to point out their flaws, to put them under eustress, and to reward them when they grow. And in this light, the ritual character of the D&D game which I found so disturbing seems perfectly natural.
I apologize, DMs, for adding "spiritual guidance" to your prep burdens (using a loose, materialist definition of "spiritual", as in "of or related to the human spirit; ie, morale, emotion, and character"). I certainly do not consider myself qualified in that department.
All this still doesn't answer the essential question of "is it worth it?" If you're spending 12 hours a week on prep+game for four people, you're looking at effectively three hours each of time investment per week, for a very ill-defined return. Could you do better by just... unstructured socializing with these people for the same amount of time, and talking about their problems? I think maybe no - the game and the ritual provide a context where failure and self-examination are tolerable, where we can get at hard truths precisely because we're all lying.
The ultimate measure of the game, from this perspective, might be "do people outgrow your games, and go on to live happy lives?" I spoke with one of my old players recently, and he mentioned that he has come to see RPGs / storygames as a form of "group therapy." I don't know that I'd go that far (those are very soft words), but I think it's reasonably consistent with my meaning.
I don't mean to say that TSR D&D was intentionally designed as a training program. I think training is a welcome, desirable side effect which can justify the activity; for another example, consider The Morals of Chess or mahjong's correspondence with reality. Norbert Wiener, in The Human Use of Human Beings (one of the seminal books of cybernetics), discusses the function of play, and argues that generally, play is training in vertebrate species, who come into the world with much more plasticity than eg invertebrates; you will never see an ant play. We humans play as well, and for the same reasons: training and pack-bonding, in preparation for the difficulties of life.