Okay, here's an example of what I was talking about the other day: a megadungeon made up of lots of minidungeons. The difference, for me, is that my normal megadungeon would have lots of long corridors and generally intersecting areas. Nasty monsters would lair in a certain area, of course, but there wouldn't be as many areas that were structurally distinct.
On the map below, there are basically three mini-dungeons that articulate at certain points. This reflects the fact that they are all on the first level below the sewers of an ancient city on the Lost Continent. Over a couple thousand years (or 10), many cults, government agencies, guilds, etc. have carved out their own areas, often using an abandoned area as an entryway.
In meta-gaming terms, my players can explore a certain area and end with what we have started to consider the perfect mixed emotion: the feeling that you found a new area and explored it's eldritch secrets, while also realizing that the dungeon is far larger and stranger than you could imagine, and that there're still countless hidden hallways and forgotten chambers deep below.
Anywhoo, as I mentioned before, I'm sure this is not new to some. It represents a nice insight for me, however, in that it's one of the best types of knowledge: those that come from actual play experience and communal realization.
Here's a crappy map: