One of the things that draws fire from dungeoncrawl adventure critics is the reasonableness of dungeons filled with a mish-mash of monsters, treasure, and confusing architecture beneath some abandoned keep or in a network of caves (and mega-dungeons get it worse).
Mad uber-mages like Halaster Blackcloak may find it fun to move around rooms, corridors, monsters, and traps inside their dungeon, it stretches plausibility in an extended campaign to find yet another mega-dungeon somewhere. ("By the beard of Zoroaster, we should make psychological screening of mages at Hogwarts a requirement!" "But we get so much tax revenue from adventurers who clear their dungeons out periodically...")
While sites like dungeonaday.com have a rationale for the irrationality of dungeons ("No wonder they're all so aggressive -- there's no place to relieve yourself here." "Don't worry, this level has a gelatinous cube.") It can't hurt to have multiple ideas to justify the dungeons and megadungeons we love so much.
So I'm going to start posting ideas filched from other possibly lesser known RPG settings that might help DMs and Players like me who care about that sorta thing.
Update (Feb 2011)
Here's a list of what I've done so far:
But it also occurred to me that I should take a look at what other "mashups" folks have done with D&D settings, especially with Mystara. Of course one that comes to mind is Tim Brannan's Mystoerth work on his blog. That's some good reading, especially since I also like to pilfer from Greyhawk for my D&D fantasy games.
Oddly enough, I prefer to keep Forgotten Realms primarily its own creature. I don't know why, but it could be because in my mind Mystara and Greyhawk were more prone to the early "mix & match" approach to settings, while the Realms -- to my mind -- have a particular canon that must be adhered to.