The cool thing about nethack is each game is VERY different. You start each game with a different variety of weapons/magic/skills that make your initial level choices interesting. Then once you get to a certain level there are a ton of sub quests and sub areas that, again, change every time with some areas have some basic flavor.
And the gameplay is pretty limitless as to what choices you can make and suceed. Do you want to be nice and buy things from a shopkeeper or do you want to train your pet to steal items out of the shop or do you kill the shopkeeper to get the stuff or do you teleport out of there or do you dig your way out of there, etc. Each issue in nethack can be solved in a myriad of ways.
It's hard to explain unless you get hooked. Of course the best one is to get the wand that transforms your items/pets/yourself into other items. I love to zap my pet and end up with a dragon, or to zap a puny peice of armor and come away with something really cool.
But hey I'll stop now

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