My main problem with nethack is the over-reliance on "spoilers" if you don't want to figure out every (sometimes rather non-obvious) mechanics in the game through trial-and-error. It's really hard to get into nethack if you're not willing to spoiler yourself, and once you start reading spoilers it's hard to know when to stop if you don't want to spoil the fun. On top of that trial-and-error in a game with perma-death can quickly become discouraging as you die stupidly because you didn't know that you couldn't levitate above a sink for some reason.

Dungeon Crawl on the other hand is a lot friendlier, in general the mechanics are well explained and you know what to expect and how things work. Contrast for instance the curse mechanic between nethack[1] and DCSS[2]. Nethack is obviously a lot more intricate but good luck figuring out all of that on your own, and good luck finishing the game if you haven't figured out most of that.

[1] http://nethack.wikia.com/wiki/Cursed

[2] http://crawl.chaosforge.org/Cursed