[vintermann]

One thing which I don't know if you've noticed (and I don't consider this a spoiler) but Nethack has level scaling. If you get levels too fast, faster than you get better gear, enemies outscale you. In my (admittedly very dated) experience a lot of the difficulty was striking that balance between exploring too quickly and lingering too long.

[Hackbraten]

Thanks. Are you referring to the dungeon level (depth) or the experience level?

[bhaak]

Dungeon level depth, mostly.

This being NetHack, an answer is often not as straight forward as it could be. Most of the time the level difficulty is proportional to how deep you are into the dungeon but there are levels where your experience level factors in as well.

[vintermann]

If so, it's changed. Back when I played, spawning of random monsters (i.e. almost all of them) was based on simply dungeon level + experience level.