Guest writer Brian Critser dives in-depth into the various roguelikes over on the Rampant Games blog, describing the mainstay titles like Ancient Domains of Mystery and NetHack, as well as variants.
What makes ADOM stand apart from the rest are its skill system, quests, an honest to goodness plot, monster behavior and other tiny little things like the random bonus you get when creating your character. There are other little things like a wilderness, talking to NPCs to get information about quests, weather and corruption.
I would like to point out again just how difficult this game is. I've read review after review, post after post about how people have had a hard time surviving in this game. I do agree with them, but can't understand them at the same time.
I was a total newb to roguelikes when I first fired up ADOM. On top of that handicap I played a rather weak character, a dark elf necromancer. I got kicked around like a ragdoll at first, but over time I started wising up on how to survive with such a weak character. I was kicking butt and taking names for awhile until, of course, I got too cocky and was killed for my stupidity. Poor Rincewind just couldn't handle those Iron Golems the first time I encountered them, but I did learn a lesson when dealing with Iron Golems.