I think it's hilarious (and somehow appropriate) that most of this thread is arguing about the provenance of THACO.
Less eloquently than Mearls, I'd say Classic D&D is comprehensively and pervasively
weird, and immersion in the weird is indeed a
unique experience.
* The books and boxes were weird. The physical product didn't look like anything else in my house.
* The art was weird.
* The fonts were weird.
* The language was (very) weird.
* The rules were weird.
* The creatures and spells were weird.
* The names were weird.
* The environment of play was weird.
* Actual play -- what you did when playing the game -- was super-weird.
The weird rules -- along with all the other weird elements -- definitely contributed to the pervasive weirdness of it. The game is such fertile ground for nostalgia precisely because of this pervasive weirdness. You ditch some of the weird elements and keep others, you lose some of the weirdness, the pervasiveness of the weirdness. D&D with slick art isn't as weird. D&D after all of its unique genre elements have been subsumed into pop culture can't be as weird. D&D with more sensible and accessible rules and language isn't as weird. D&D where actual play is a "heroic quest" or "adventure path" or "epic story" isn't as weird as rounding a corner in the Caves of Chaos and getting turned to stone by a medusa.
Immersion in the weird is part of the Hero's Journey, but not all of it. It's the part that pretty much every Classic D&D game had, even when most of them lacked most of the other elements of the Hero's Journey. Immersion in the weird is a unique experience, and the comparison to the hard-to-describe experience of a horror movie is apt.