Riffing off some recent posts I've read - how big does a dungeon need to be in order to be a "mega" dungeon?
First, I think it needs to be a . . .
Discrete Area. A megadungeon is a fundamentally interconnected underground area. I think if you blow that up too big, and spread out the connections too much, it goes from "megadungeon" to "series of dungeons" (Caves of Chaos) or "underground wilderness" (ala D1-2). Those are cool, and great campaign settings, but once it's "travel for a week to the next encounter" it's really just a wilderness game with travel via tunnels.
Second, size matters. I think it needs to be big enough to support:
Repeated Play - You need to be able to go back to this place over and over. It's not a question of "can't be cleared out" but rather of "supports continuous play." You should be able to use the megadungeon as the basis for many sessions.
Cumulative play - the play should be cumulative, too, in that what you do to change the environment changes the environment for future trips. Re-popping is your enemy. Restocking is your friend.
Diverse Challenge Levels - It needs to mix difficulty ratings within the same dungeon, all on a large enough scale that those challenges also take repeated play. There needs to be the risk of going into too dangerous of an area, but also both easy stuff and hard stuff mixed together so it can hold up to repeated play.
End Points, But No End - there needs to be things you can accomplish in it, that says you did more than farm gold and grind monsters. But not a single end point that finishes it as an adventuring environment. A dungeon often has an end (a demi-lich, say, or a boss fight with a dragon) but a megadungeon has accomplishments.
Even Castle Greyhawk (mark I) had an end point - the wizard down on level 13 you could meet. But it didn't finish the dungeon as an adventuring environment, since he wasn't a load bearing monster. It was still a place you (even the same PC "you") could adventure in.
Things I don't think matter so much:
Specific Room Count - I don't think it's worth getting hung up on room count. My first two-three levels could swallow most or all of Stonehell, but that's still a megadungeon. It's not the room count that matters. Lots of rooms helps, but there isn't a numerical threshold. Level count, either - Barrowmaze is one big level, and it's pretty mega.
Multiple Groups - doesn't matter if one group of players or one group of PCs do all of the adventuring. It's nice if the dungeon can handle more groups, but that's just a subset of repeated play, above. It shouldn't matter if it's the same group or not.
Game Mechanics - I don't play a class-and-levels game, so I have zero concerns with level advancement, appropriate treasure distribution, or challenge ratings by level. None of that defines "megadungeon" for me, and I think it's too game-specific to worry about those things . . . if you make them part of the definition of a "megadungeon" then you're limiting the definition by the game mechanics. The dungeon should be mega or not based on its non-mechanical features.
Factions - I also don't think factions matter so much. It's a nice feature, if monsters come in groups and you can negotiate with them or pit them against one another or something. But If you have a dungeon that meets the rest of the requirements above, does it really matter if there is only one faction? Moria was pretty megadungeonish, and what I remember was a bunch of orc-kind and a boss monster, and that was that. You could spend a campaign clearing out Moria without ever needing factions.
For me, it's the size (it's big, and can hold a wide variety of challenge levels) and scope (can handle repeated play natively) that make a dungeon mega.