My answer to an r/osr question about coming from D&D 5e to OSR:
Having done both in the last few years:
5e feels like slightly too much game for me. I’ve no interest in ‘gamist’ tactical combat, which is Hasbro’s preference — 4e was chess++, and much 5e (especially on Roll20 and equivalent) tends in that direction. B/X wants you playing fast and loose, maybe with a map but definitely with the ‘fiction-first’ approach that people misidentify as a storygame/’NuSR’ innovation. Once you’re playing on a grid you’re too far from the fiction for my tastes. Others feel otherwise.
False precision creates ambiguity and tension, opportunities for haggling — little things, but the game is full of little things. 5e is infamously falsely precise: look at how many stupid fucking clarifications and FAQs there are for what’s supposed to be a rough’n’ready adventure game ruleset. Why is Jeremy Crawford forced to spend so much time on Twitter assuaging the anxieties of young players? (The real answer is partly generational, and therefore considered rude. Let’s not.)
The 5e skill system feels like it’s a few pounds overweight; it imposes on play clumsily at times. A small example: the other day my dwarven cleric had a chance to recall some geological knowledge during a dungeon crawl. The DM wanted to call for a skill check to adjudicate that moment — but felt that INT/History (the ‘stonecunning’ class feature) didn’t quite make sense. The class feature is written juuuuust prescriptively enough to get in the way, like the spell descriptions. It worked out fine on the night of course, slowed play for 15-30 seconds or something — but the night is only so long, you know? Those decisions pile up. (FWIW, I’d have ruled it a roll-under INT check with Advantage, or an auto-success, but our DM is good and it’s his table.) There’s not much point in playing 5e if you don’t use the rules, even when they say both too much and too little at once. So the DM had to think about the right skill check, because the rules insist that you think about it, and we had this brief moment of uncertainty about a rule that doesn’t need to exist in the first place.
5e is too often full of such moments — when you try to give the character sheet what you think it wants instead of just flying. In my experience, B/X isn’t that way: you say yes, or roll the obvious ability check (perhaps using the hidden skill system), or roll the obvious-with-a-bit-of-practice saving throw. To my eye, the stat check/saving throw system is good at catching what’s tossed to it, while imposing minimal cognitive burden during an already extremely cognitively demanding game.
Settings and situations are easier to roleplay than plots — well, ‘plots’ are arguably impossible to play anyway, cf. the early Forge insight about the Impossible Thing. When players call for ‘story’ and ‘plot’ it seems to me they actually want rich situations with legible stakes in an interesting, interactive setting, and they want their choices to matter. (Will Wright, as I recall: a game is a series of meaningful choices.) Latter-day WotC/Hasbro adventures usually fail here: they tend to be unimaginative or thin, with illusionist ‘choice’ disconnected from the setting/stakes, in boring settings. (WotC has no great writers left.) It’s no surprise that they’ve made great money wrapping 40-year-old adventures in new trade dress (and punishingly dull art). Simple rule of thumb: an adventure that presumes to tell you what the players will or must do is incorrect.
(For the opposing view, Ken Hite: ‘”Railroading” is a pejorative term for an adventure in which something is actually accomplished.’)
The old-school gold-for-XP rule is a great setup for a certain kind of adventure, because the rulebook doesn’t presume to tell you how to solve problems — it poses a challenge (find treasure) and trusts you to have (make) fun completing it. XP for combat is widely understood even by previous adherents to be merely dumb; ‘milestone XP’ is better for lame mainline use cases, but its hidden purpose is to drive players toward the pre-written, agency-denying ‘plot.’ That’s a thing I intensely dislike about 5e: its attitude toward ‘experience,’ which flows from its focus on character ‘builds’ — which the designers of the game halfheartedly disavow.
So that’s some of how I feel and some of what I think. 5e is too much game in the wrong places, and not enough in others. Its writing and art are dull and its adventures trip over themselves. B/X is lighter in the hand, quicker at the table.