[btw this should be understood, but doesn't hurt to mention now and then. It's easy to misinterpret online and spiral out of control, RPGBN ahhem. Even though I'm snarky and sarcastic at times I mean/imply nothing insulting, derogatory, or cruel. I respect and value the words and opinions of my readers and fellow bloggers. Even when they're totally freakin wrong! ;) ]
Temple of Elemental Evil
It's not quite big enough to be a mega megadungeon. It's close and what megadungeon starts out fully formed? Most posts about megadungeons I've read say they must be broad brush strokes, a starting point for DM's to make their own. They recommend creating first few levels and growing it from there. My definition of megadungeon also includes "requires, encourages tweaks & customization." ToEE is ready for growth. It gives you the first four levels (and several sublevels, as well as the surface neigboorhood). Up to you to take it off to the elemental nodes, into the Demon Webs, into forgotten levels, into the Underdark, etc.
Some deride ToEE for being plot driven and the "storypath" main event of the campaign [my paraphrase]. Rather than a nice sandbox within a sandbox that a megadungeon should be. ToEE is only the focus of a campaign if DM / players make it. I see nothing in the published module that implies it must or even should be the sole event. In fact there's a page on Other Adventures. Where it says "you may -- and should -- design and develop other adventures in the area."
Megadungeons are not static. They are dynamic, with conflicting factions, wars, and plots. Reactionary to the characters intrusions. That describes ToEE to a 'T'. There are numerous factions & plots and room for DM created ones in ToEE. The stop "re-ascension of Temple" is just the hook TSR puts into every module. It is totally ignorable.
Old Codger croaks "Temple was sealed up pretty good, not perfect. Why we don't go round there. But, I heard all the treasure was locked up with the critters. Laying there, just waiting for someone brave enough to claim it!"Works just as well with no plot other than let's explore and loot. As the players explore ToEE they may or may not stumble upon / deduce the various plots and subplots. They may ignore them. They may pack up and move to the other side of the planet. That setup sounds not dissimilar to every megadungeon that I've heard described.
Alternate plots and goals were explored in the computer adaptation (one reason it's a great game). Each party alignment had different reasons and goals for entering the dungeon (9 or so e.g. recover holy avenger and kill goody-2-shoes knight guy, trap Fungus Girl, free Fungus Girl, ransack the place for whatever loot we can and not get killed). Even those were ignorable. It was a total sandbox, do whatever you want. And that was a freaking pre-written, static computer game. A human DM would (and I did) rock that house!
It's been argued that ToEE runs out of bad guys and/or can be "finished".
ToEE has several sources of bad guy replenishment. 1) elemental nodes. 2) Elemental Chaos attracting creatures from all over. 3) insane demigod(s) there are at least three of those in play. 4) in my ToEE there were connections to D1-2 and the "Underdark" It's even implied that more baddies will come to fill the ranks (the rise of Elemental Evil is attracting creatures. There are several examples of that in the moathouse and elsewhere). Experienced DM's don't need to be told how a restock a dungeon. But, ToEE is a training tool for teaching that to newbies.
In the backstory of ToEE the temple had already been "finished" once. There's a published return to ToEE after it has been "finished" a second time. Both of which conflict with idea ToEE can be finished. Zuggtmoy and her followers are not the only power faction in the dungeon, far from it. End (or release) her it's not over (unless you want it to be), just changed. Zuggtmoy can even be a red herring perpetuated to keep the forces of chaos fighting amongst themselves and weak. Again "ending" ToEE is purely a decision of the DM/players. It is not built into the dungeon.
But, ya know, YMMV!
I think peoples recollections are clouded by past experiences. And possibly by a desire to pigeon hole ToEE so as to not conflict with their current beliefs. In similar vein I may be under-appreciating the interpretations and alterations I've made to ToEE. I may also still not understand what others mean by megadungeon which is complicated by people having various secular and religious views on what megadungeon means.
ToEE wasn't published as a megadungeon. It was a proto-megadungeon which TSR grafted some "module" facade onto. Those moduley bits are easily scraped off. It's definitely a product you can hand someone "Here are the first levels of a megadungeon. Go read mythic underworld, these posts and forum threads on running a megadungeon and make it your own!" And, if that doesn't qualify a published something for megadungeonhood then nothing does.