Telengard was a dungeon crawl computer game put out by Avalon Hill back in the early 80's. Like most such computer games of the era, it gleefully swiped D&D's mechanics.
Each level of Telengard had a teleporter device that would take you to another level of your choice. I spent hours sending first level adventurers down to level 50 in hopes of stumbling across some unguarded high-power magic items and teleporting back up to level 1. It rarely worked. Usually I'd find a level 50 wraith or someone that would kill my impertinent newbie PC.
Which brings me around to a simple premise: D&D was designed for dungeons. Sounds obvious, I know. But hear me out. D&D was not designed for ecologically-sound underground lairs. D&D was not designed for adventures with plots and stuff that happen to be set at least partially in subterranean labyrinths. You can do both those things with D&D, but the game is ideally designed for the big, sprawling megadungeon where Orcus lives on level 20 or whatever.
In a megadungeon the PCs can strategically gauge there risk based upon their own level and the levels of the dungeon. Say we're a pretty big and tough party of 10th level adventurers. Do we putz around on the upper levels of the dungeon, pimp-slapping kobolds til they cry and stealing their copper pieces? Everyone will come back alive, but the loot and xp haul will be low. Or do we go to level 10, where we can expect a "fair fight" and earns more rewards if we survive? Or do we spend some gold on extra scrolls of invisibility and silence and try to sneak into the lich-king's treasury on level 15? If we make it out with the liche king's crown jewels it will be worth the additional risk.
Now, the scenarios in the preceding paragraph assume a lot. Assumption one is that it is relatively easy to traverse from one level to the next. A good megadungeon should allow for lots of flow from one main level to another, with only certain sub-levels isolated. Also built into the scenarios is the assumption that the party doesn't just show up to the dungeon and wander willy-nilly. Superior dungeoneers reconnoiter. They map. They plan. They interrogate or charm monsters to get information. Certainly you can slop your way through a dungeon crawl if you want (and that can be lots of fun) but if you want to do a good job of plundering the dungeon you take planning and reconnaisance seriously. Under this approach some dungeon expeditions should be strictly scouting missions. Fighters should wear leather armor to increase chances of surprise and the party flees from most encounters. Finally the biggest assumption is that the DM is on his toes. Maybe he doesn't have the map and key to level 15 complete, but he knows enough to that he can have the ki-rin on level 8 tell the party that the lich-king's treasury is down there.
But if you can put that all together, dungeoneering won't simply be an endless series of mindless slogs. For example, say you've got a successful mid or high level party. You map of level 3 is incomplete, or maybe you just want to check for secret doors that might have been missed back near the beginning of the campaign. Sending a Wizard, a Lord, and a Patriarch to do that seems like overkill. But what about sending the henchmen? Those who have henchmen get an opportunity to score some xp for their sidekicks, while anyone who doesn't have henchies can start a back-up PC.
Now assume that things go bad for the little spuds and everyone's favorite henchmen end up captured by a wandering monster. Time for a rescue operation! A few sessions ago didn't we find a series of prisoner cells on level 7? Let's send the big guns down there and hope that's where the baddies took our sidekicks.
"Man, is there anything Jeff CAN'T do when it comes to gaming? This guy is like a critical 20 every roll. Jeff can bite the heads offa five game geeks, including their sorry-ass DM, and spit 'em into a large duffel bag ONE AT A TIME!...that's just the kind of messed up bastard he is! You think yer a gamer, punk? Well..do ya? Jeff will depants your weasel-ass right in front of your grandma."