I started with the Basic Set in 1979 when I was a 12 year-old Tolkien-o-phile and then quickly moved on to AD&D in 1980. The 1E books were written by adult wargamers for a literate demographic of bright high schoolers and college students. I still have my near-pristine PHB, MM, DMG, D&D, and FF. Why did I keep them all of these years even though my actual playing of D&D waned considerably?
Two reasons: 1) AD&D was an important childhood touchstone; 2) the books were aimed at an educated, reasonably sophisticated demographic, not a grabasstic hoard of zit-popping button mashers. From time to time I still pull out one or more of my books and read through them for sake of entertainment and nostalgia because they were that well written. The same cannot be said of the lifeless dreck WotC churned out for 4E.
Maybe I'm just a zit-popping button masher, but I've never felt any desire to read (say) the AD&D Monster Manual for sheer entertainment. It's full of long lists of stats, plus multiple prose paragraphs in which are buried long lists of stats.
Here is the 4e MM entry for spectres:
Insane and unfettered by the memories of its past life, a specter exists only to snuff out the living. It appears as a ghostly, twisted apparition.
That is as interesting and informative about a spectre as anything in the AD&D MM or the Moldvay Expert rulebook (I don't have the AD&D entry in front of me, but my recollection of it is that it mostly discusses the spectre's immunity to various weapons and spells, and the level draining effect of its touch).
Here is part of the 4e MM entry for spiders:
Spiders are sacred to the evil goddess Lolth. Long ago, before she became the Demon Queen of Spiders, Lolth was a deity of fate who wove the strands of mortal destiny; it’s said she created the art of weaving after watching spiders make their webs.
I don't remember anything that interesting in the spider entry in the AD&D MM. (Phase spiders, the most interesting part of the entry, are barely elaborated upon.)
The only really interesting entries in the original MM that I can recall concern demons and devils (obviously!), the tribal names for orcs and hobgoblins (which in the case of orcs went on to form the basis for the holy symbols of their pantheon), and the sahuagin entry. The only problem with the sahuagin entry is that it builds up this rich social structure for a humanoid race that is niche at best. Imagine if we'd been given that sort of detail about hobgoblins, or some other more mainstream humanoid race. Perhaps something like this:
Goblins form tribes, each ruled by a chieftain. The chieftain is usually the strongest member of the tribe, though some chieftains rely on guile more than martial strength.
Hobgoblins rule the most civilized goblin tribes, sometimes building small settlements and fortresses that rival those of human construction. Goblins and bugbears, left to their own devices, are more barbaric and less industrious than hobgoblins. Bugbears are dominant in a few mixed tribes, but hobgoblins tend to rise above their more brutish cousins unless severely outnumbered.. .
Hobgoblins once had an empire in which bugbears and goblins were their servants. This empire fell to internal strife and interference from otherworldly forces—perhaps the fey, whom many goblins hate.
Hobgoblins developed mundane and magical methods for taming and breeding beasts as guards, laborers, and soldiers. They have a knack for working with wolves and worgs, and some drake breeds owe their existence directly to hobgoblin meddling. All goblins carry on this tradition of domesticating beasts.
Given their brutal magical traditions, hobgoblins might have created their cousins in ancient times: Bugbears served as elite warriors, and goblins worked as scouts and infiltrators. The disintegration of hobgoblin power led to widespread and diverse sorts of goblin tribes.
(Which, for the curious, is from the 4e MM. And even uses "whom" rather than "who" when the grammar of the sentence calls for it. And also paragraphs.)