So complex your brain hurts just looking at it.
After world generation, which may be several minutes or much longer depending on individual world settings, choosing an embark site and individual equipment for your initial party, you are set in the wilderness with a handful of dwarves under your command, and here is the game's first problem.
Although they will automatically drink and eat from their supply wagon, the dwarves are not self-sufficient and will soon starve to death unless you order them to find shelter and farm for themselves. All well and good, yes, but every order and command for your dwarves is buried beneath varying levels of sub-menus, some of which appear to be redundant at first glance, some have abbreviated names and some serve no immediate purpose. Performing any sort of meaningful task is a nightmare of micromanagement, and Heaven forbid you want to organize a military.
Tales abound of improbable combat scenarios and silly contraptions encountered or manufactured by players, but in reality you may play the game for several weeks without a single dwarf dying from anything less than a freak accident that would have been unavoidable anyway. This can be understandably disappointing to some neophytes who heard of the game through a story of a dwarven wrestler who was able to punch goblins so hard that they flew in to a tree and exploded. Quick payoff is not one of this game's strengths.
One point in this game's favor that I must grudgingly award is that the default ASCII graphics generally do a rather good job of conveying what is happening at a given moment in the game world. A completely new player can tell what each symbol represents with a small amount of memorization, and experienced players of roguelikes will be glad to know that the game mostly follows standard ASCII-graphics conventions (a 'c' represents a cat, an @ is a human, 'g' is for goblin, etc).
However, due to impenetrable controls (that require hours of poring over a wiki to understand), potentially low payoff for hours of dedication, and the fact that a player may play the game for a month or two and only experience a small fraction of the content available due to plain bad luck, means that Dwarf Fortress fills a relatively small niche deep within the already niche genre of simulation.