I've actually been writing a multiplayer roguelike so I may take a stab at some of these.
if you had to wait for others to finish their turn or if the turns had a time limit, it would take much longer than couple of weeks for anyone to finish.
I don't think there's any way to have a turn-based multiplayer dungeon game with a significant amount of people in it. At some point someone lags and the game dies. If you decide to have a cut off point (like 5 seconds per turn) then it just becomes a really slow real-time game. People hate it and stopped playing almost immediately during play testing.
this is what many people forget. the interface as it is, is suited for a single player game. like instead of pressing a button and typing in a number to wait for 100 turns would you rather wait half an hour? all the game mechanics would need changing.
Pretty much on the money here, I've had to re-evaluate almost every mechanic, especially the sleep/paralysis ones. The good news is that if you play in a team you are suddenly a lot more resilient to these effects, your team becomes your shield.
thing is, nethack is TURN BASED. changing it to a realtime game doesn't quite work out simply and in the end it is something totally else.
I agree that it would be near impossible to port all the NetHack mechanics verbatim. You could probably make something for a small team of four people or so, who are friends and talking on teamspeak or something, but not a game with hundreds or thousands of players.
and in a little while all the levels would be digged up. of course, you could make them bigger than the screen but that would be totally changing the game mechanic again.
I did playtesting with destructable dungeons, it's a nightmare. You just can't let people dig holes in the floors, walls. Perhaps the only way it would work is if it literally took hours to dig one square. I experimented with allowing you to dig a path with a pick, and letting the dungeon heal itself over time, it works OK but doesn't really add much to the game.
If you want to check out what I came up with check out Squadhack. It's in an early alpha at the moment and many things don't work but due to the graphics it will be familiar to many NetHack players.