Hiya!
Hmmm...guess my old skool upbringing is rearing it's old grizzled head again.

To me, a "mega-dungeon" is a dungeon that comprises at least 13 sheets of 8.5"x11" 4/inch graph paper sheets. Each sheet is a level. Each level can have any number of 'rooms', but each level should have a single over-riding theme ("The Orc Enclave", "The Corridors of the Watchers", "Caverns of the Troglodytes", "Dusty Halls of Mogg", etc). Overall, each level that has a level both above and below should have some notation as to why denizens don't go from one to the other "in general". At the bottom of the lowest level of the dungeon there should be something "Campaign Worthy". An Ancient Gold Dragon held in stasis, or a permanent Gate to Asmodeus' City, or something equally "WOW!".
That's a "mega-dungeon" to me. I know the more common idea is "a few levels with 15 to 20 rooms"...but that just seems like a dungeon. Some are small, some medium, some large, some very large; but to be "mega", each level needs to "work" with the others (the "Theme" thing)...and at the same time be more or less 'self-contained'. So, to me, the Temple of Elemental Evil is not really a mega-dungeon. I'd put it as a "medium" dungeon. If the DM adds/adapts the elemental nodes to play a more important part/link, then we hit "kinda large" dungeon. Close...but not close enough.
Undermountain? Yeah, that has POTENTIAL to be a mega-dungeon; as it stands, it's just a few big-ass maps and a handful of encounters. If a DM "stocks" it, and keeps to the idea of "Themes", and adds another two or three poster-sized 'levels'...and finishes it off with some Campaign Worthy "end/bottom/conclusion-if-successful". That would be a mega-dungeon.
^_^
Paul L. Ming