My group is just starting to traverse WLD, so I'll have a new perspective within 3-4 years or so.
Actually, there were 2 mega-dungeon experiences before WLD for me: Undermountain, which just fell flat on its face not long after it was started (having low-level PCs bump into the Balor room just wrecked it), & Night Below, which was a pain to endure, mainly because it was a standard 2nd AD&D module which had a bunch of non-standard PCs running thorugh it (way too many sorts of elves of some sort w/ thieving/magery, not enough warriors or clerics, and nil on dwarves or the like). Plus, the "all at once" theme made that a real hell. On top of that, irregular attendence & a changing group roster really played hell on the game, & I daresay that it killed any real excitement/fun it could have had.
I think that the key thing really needed is, if the PCs can't leave, then there should be some sort of reliable fallback point for rest & recovery (and, maybe at the least, a friendly faction or two to interact with). As previous posters have said, the original D&D dungeons of renown (like Castle Greyhawk) were taken in doses--they were expeditions into the place (with resupply & deposits back in the Overbright), & not 1 long uninterrupted journey. Putting a
time crunch on top of it made things absoultely worse.
It seems (IMHO) that the Mines of Moria/Khazad-Dum really kicked off the notion of the massive underground complex to delve in, but even that scenario was just getting from point A to point B, rather than mapping out the entirety of the place. However, that complex made sense because it was a realm, a city, heck, a small nation underground. It made a lot more sense than the massive dungeon-themed (lethal) Danger Rooms that some dungeons take shape as.
BTW, since I'm just starting WLD, could anyone please warn of spoilers for WLD in their posts, or at least avoid mentioning spoilers?