I dunno I guess I just tend to prefer adventures that have strong plot and fascinating villians. Stuff like B2 just doesn't interest me because I see a stat block for a Dark Knight...but there's little/no detail into why he's in the caves, whats his motivations/alliances. He's just another guy in a room. And I know this is a fantasy game, but dungeon ecology is pretty much lacking in 1st/2nd ed. Its more of a room-room-room-boss-town experience, and I'm sure thats fun for some people...its just not my cup of tea.
I obviously don't mind if it's not your cup of tea. But you're close to touching on the differences here.
First, old school adventures are not about having a "plot", especially not a "strong" one. To most old schoolers, a "story" is not something you have during play... it's something you tell about the play session once it's over. As I believe the great Papers&Paychecks (OSRIC guru) said: "We explore dungeons not characters."
Old schoolers don't really view D&D sessions as if they were movies. You know, the best Star Trek movie is Wrath of Khan not just because it's the only one with cool ship to ship combat but also because it explores the character of Captain Kirk. If I'm the moviegoer I want the movie to explore Kirk... but if I'm Captain Kirk I don't want to explore Captain Kirk, I want to explore outer space! We the moviegoing audience might want to see the character explored, but the character himself wants to explore outer space not inner space. And in D&D you get to be the character. So let's go exploring!
Second, one of the reasons that extreme detail was not given in the earlier products is because you were expected to
make it up! Why is this "Dark Knight" character here? I don't you... you tell me. Maybe he's under a curse. Maybe he's trapped. Maybe he's running from the authorities of the Realm. Maybe he's running from the Furies, and they can't find him while he's in the Underworld. Maybe he has amnesia. Maybe he's just a weirdo. But unless it's relevant, we don't really need to know where he poops or how his mother treated him as a child. We need just enough for him to be an interesting and cool challenge to throw in the way of the players. Also, old school games often operated under the assumption that you were going to be rolling for NPC Reactions. Maybe the Dark Knight doesn't want to fight. Maybe he wants to challenge somebody to let them pass. Maybe he will give up his magic shield if they win (maybe he's got a lot of shields already on his wall). Maybe he will join the party... and maybe he will betray them later.
The DM can make up all that kind of stuff. He can tailor it to fit his campaign and his group. If you're all engineering students, maybe the "Dark Knight" needs help repairing a Dwarven mining device (break out the slide rules, boys).
Part of the "old school way" is to give the player or DM something partial and see what they make out of it. Look at how characters are made: you roll randomly for your attributes. What if you roll high Strength and low Constitution? Maybe he's a wiry character, or maybe he's big and strong but has a weak immune system. That's part of the challenge: take these raw numbers and make them into something imaginative and fun.
Third, remember that dungeons were originally called "underworlds". You suggest that they don't make sense. And you're right... they don't. They're the city of Chaos, the "anti-city", which stands as an opposite and counterbalance to the organized and logical realm of Law, the city. Ever notice how, in Greek myths, the monsters and crazy cultists and strange visitations all usually happen out in the wilderness or underground? That's because you're safe in the city, where reason and order prevail, but outside the city walls you are in the realm of the mythic. It's basically that way in D&D, too. Especially with regard to the "dungeons" component of "Dungeons and Dragons". The dungeon or underworld is the opposite of the civilized realm, and things work differently there than they work in the realms of men.
Well, that's enough for now. Just a few points for you to munch on. Again, if you're into a different kind of experience, that's fine. But I hope you come to see that there are reasons why things were done the way they were, and why those of us who still run and play in old school games continue to do so. It's not because we're unable to keep up with change (like, we're on the internet and all) or that we have "rose colored glasses" or something. It's just a different approach to gaming which aims at delivering a different kind of experience.