Well, let's at least include the Ed Greenwood quote that (I assume) started this. The source is Part 2 of his classic Nine Hells articles, in Dragon #76. The quote is taken from his description of Nessus, the lowest layer of Hell:
"It is said that the nycadaemon Daerith once appeared unannounced over the lake of Cocytus and flew toward the palace of Asmodeus, and before it reached the crags about the lake was destroyed utterly, torn apart bodily by pit fiends on patrol. With such guardians, it is small wonder why the mysteries of the plane are so many, and the visitors who return whence they came so few."
More has been written on Nessus in later sources by other writers, all building on this article. 2e and 3e confirmed that Nessus is a scary place indeed - maybe the scariest of all places. Until 4e arrived, Asmodeus's fortress of Malsheem was described as the most massive citadel in the entire multiverse: 40 miles to a side, and holding millions of devils (primarily pit fiends and cornugons/horned devils). At least one source described this army as being held back for something truly apocalyptic, and that the diabolic forces committed to the infamous Blood War against the demons of the Abyss were only a "drop in the bucket" - a decoy force to distract the enemies of Hell from the real power at Asmodeus's command.
Given the power of a pit fiend in all versions of the game, that's a heck of a concept. Millions of pit fiends... in a fortress 40 miles to side... held back for a mighty conflict that makes the Blood War look like a side-show. That's always appealed to me, frankly. At a time when some players/DMs view the archfiends as beatable monsters, and their homes as mappable dungeons, it implies a level of scope that is beyond the wildest imaginings of even the most Epic level character.
You see some of that same scope applied in a few other areas. 9,999 ice devils guard the only approach to Nessus from the 8th layer of Hell. Khin-Oin, the fortress of the daemons/yugloths, is a massive spinal column, miles long, half above-ground and half below-ground. Githyanki build their cities on the floating bodies of dead gods. There are these... things... floating in the astral sea called draedens. If you're not familiar with BECMI D&D, you don't even want to know what they're about...
That's the kind of thing I'd really like to see more in Epic-level D&D adventures. Settings and situations that are just impossibly huge and over-powered, and require more from the PCs than just rolling for initiative and dropping a couple of 29th level attack powers. Your high-level PC can have a vorpal blade and a hammer of thunderbolts and a meteor swarm, and it's not going to matter one little bit when you're trying to take on the pit fiends on patrol. Or when you try to get through their million-or-so-buddies guarding that castle over there which is large enough to drop the entire city of New York into it... 80 times over.