One thing I've been enjoying about my megadungeon is the cumulative reactions of its denizens.
As the PCs have rolled through, some monsters have reacted differently.
The gargoyles are pretty much cowardly bullies. When it was clear the PCs couldn't kill them, they pretty much just piled on whenever they could. Yay, a fight! They'd attack, get beat but get well, and attack again.
Then the PCs dealt them a harsh lesson, and that was that. They've been avoiding them ever since.
The otyugh keeps trying to tempt the newbies with visions of treasure in its muck pile, but otherwise avoids the group. They made it pretty clear they're more dangerous to it than it is to them, and they come in big packs.
The lizard men kept ramping up their tactics and their aggression, and shortened their time to call for reinforcements. It didn't help in the end.
The orcs keep doing what they're doing. They took some losses, but there are clearly more of them and their reactions have been scaling up but also they aren't rushing in aggressively against the PCs. They tried that 2-3 times, and ended with a carpet of casualties.
That gnome is still active, clearly, and he hasn't tested his luck against the PCs since they saw through his (admittedly thin) story about being friendly. Well, he was friendly, but he's apparently a wanted criminal, and his traps annoy them, so . . .
There are others, of course. All of them change their tactics and their approach to the PCs based on their results. Not all of these will hold for all groups - new delvers might want to worry the gargoyles won't mistake them for the current group and run. But in a way the dungeon is always different. The orcs from the first few sessions aren't the orcs that are there now, even if the group that comes through is new to the dungeon. It's a living place, since I keep running it.
I remember James Maliszewski mentioning this about Dwimmermount - but as far as I know, it was a re-set dungeon each time, just run differently. I'm doing more or less what I know about Greyhawk - it's one dungeon, one continuity, for everyone. This helps keep it lively for me, too, since I get to just have monsters react based on what's happened to them so far.