I don't know about NetHack. I started with Hack back in the late 80's, and have played that then NetHack off and on since, usually picking it up for a day to a few weeks then losing interest. Never finished the game. I'd usually play until I got a guy down pretty far with a great kit, then when he inevitably died from something stupid, I'd be annoyed and lose interest again.
It's a good game, maybe even a great game, but it's not a perfect game and it's not the best game ever. Too much of it is just not fun.
The major design flaws in my mind:
* Once you hit the labyrinths and have to deal with the wizard following you around, it just becomes a grind. A little bit of a grind in order to achieve something afterwards is fine but when a game becomes work then that is not.
* It doesn't give you a fair way to figure out what to do. A lot of the actions required to finish the game are neither hinted at nor intuitive.
* It's too repetitive. It doesn't exercise my mind much; you just do the same things again and again.
* It's too time-consuming, and frequently unnecessarily so (which goes back to the repetitive point).
Anyway, just my thoughts.