Instead of happening entirely inside dungeons, ADOM has an overworld, featuring cities, wilderness encounters, and of course many dungeons. It also has more of an overarching story than Nethack (lots of NPC's, quests, moral choices, etc.) and an interesting corruption mechanic, with ChAoS slowly taking over the world and corrupting your character with mutations.

I personally prefer it to Nethack as it's more varied, the ASCII UI is better (it also has tiles but I'm and oldschool player and use ASCII), the story is richer (and allowing for lots of roleplay, feeling more like an RPG), and it relies less on spoilers. Although to each their own, of course.

For me ADOM is the best maximalistic roguelike, Brogue is the best minimalistic roguelike.

By the way, just in case you don't know, you can try it for free. The paid Steam version mainly adds configurability of things like disabling permadeath, disabling hunger and stuff like that, and it is also updated more frequently; but the free version has the full world and the core roguelike experience. There are also servers where you can play the free version in ASCII mode online through ssh and spectate games, check http://ancardia.uk.to/ (UK) and http://ancardia.us.to (USA).