Different players, different expectations.
I find it hard to manage a large dungeon unless it's well-designed. IMO, a well-designed dungeon is the opposite of Keep on the Shadowfell, which had a bunch of rooms crammed with monsters, each right next to the other. PCs really should be provoking some really large, unbeatable chained encounters.
I think a better-planned dungeon would spread the occupied areas around. To give a brief example, if the dungeon is divided between goblins, kobolds and orcs, who don't like each other, rooms in the "contested area" should be empty of living things (unless they're random dungeon monsters like cloakers, etc) and movement in those areas should be fairly free for invaders. Of course, those areas still have random encounters, as each faction attempts to claim those areas.
Each occupied area should be clearly marked off (good borders means better neighbors, less fighting, and of course it's a way to communicate with players). For instance, perhaps the border zone between goblins and kobolds is a rift or river that no one can (or wants to) live in. The goblin zone is marked by kobold skulls, and the kobold zone is marked by goblin skulls. (Young warriors frequently steal back trophies, prompting little skirmishes.) The occupied areas could be separated by considerable distances or dungeon levels, driving home what areas are what.
Each occupied room wouldn't have a full encounter. There's no need. PCs who use stealth and kill quickly might take out a bunch of relatively weak encounters in a row, whereas PCs who don't bother to learn information about the dungeon and loudly take on a room could find themselves fighting three or four weak encounters... which is the equivalent of one or two strong encounters. Do it wrong and you could take on all the kobolds at once. Even if they're not played as Tucker's kobolds there's a challenge. If they are played that way... there will be something other than goblin skulls decorating their borders the next day.
One of the big problems with a megadungeon is rest and bookkeeping. My favored dungeons are much smaller. I don't like the thought of a 30 encounter dungeon, as it's mathematically impossible for the PCs to do the whole thing in one day. In a megadungeon, PCs have to rest in the dungeon, and this could end in anything from Rope Trick abuse to PCs messing up and getting ganked in the night. Getting a good rest is challenging, and in a dungeon it should be, but sometimes that challenge can be entirely removed.
Pacing can be a problem. PCs cannot wipe out a megadungeon in a decent period of time, and often know little to nothing about the dungeon when they enter. If they're just there for treasure, they can take as long as they need, abusing the 15 minute day. On the other hand, if there's a plot, it's a challenge to get them to know it. Old Dragon Mountain was, I thought, a good dungeon at first. Much better than Keep on the Shadowfell. But there was no time limit. As far as I can tell, the PCs have as much time as they need to kill the dragon (or rather, they will probably think they have as much time as they need, with the similar impact on behavior).
