ADOM is obviously heavily inspired by Nethack, and includes many of the same, or similar, features. But it's its own (closed) code base, many of those features have twists to catch the unweary, and there are plenty of new features.

The ultimate impression is of a game not quite as fair to the player as Nethack, but still interesting for its own sake. One example of that unfairness is the behaviour of oft-killed monsters. In Nethack, if a lot of a given type of monster is killed it becomes "extinct," and more will not be generated by random means. In ADOM, monsters that are frequently killed instead become stronger to compensate. As a result, killing hordes of enemies in those frequent overworld encounters will make further encounters much more dangerous.

ADOM's overworld (a rare feature in roguelikes) contains multiple dungeons, some with interesting quirks. There is an extensive quest system whereby NPCs can ask favors of the player. These quests are hard-coded and the same every time, although the ones offered can vary depending on what the player's done and his alignment, and the specifics may change. (One character, for example, will always ask the player to kill a certain kind of monster as a quest, but which monster that is will be randomly selected.)

One of ADOM's signature features is its corruption timer. This ultimately fills the role of food in Rogue and Crawl, a way to keep the player moving and lend strategic importance to efficient processes. In ADOM however, the way it works is that the player is absorbing corruption radiation as game time passes, and at an increasing rate at the game continues. When the radiation passes certain thresholds the player mutates, gaining some weird new characteristic. Some of the mutations are good, some are bad, and a few are extremely troublesome. If the player's corruption counter gets too high he devolves into a blob-like creature and loses the game. There are also ways to gain corruption artificially, and there are ways to turn back the clock, although those are either quite rare or difficult to pull off.

ADOM's development has trailed off in recent years, as author Thomas Biskup has worked on new projects. It still remains a popular game however, has its own Usenet group, and is considered one of the "major" roguelikes, along with Nethack, Angband and Dungeon Crawl. It's a fairly advanced game, and so I can't really recommended it for roguelike beginners, but it's got considerable depths. It might be worth trying simpler games like Rogue and Crawl before moving on to this.