I spent quite a lot of time playing ADOM, which is a roguelike like Nethack, and I can attest to enormous complexity and devilish difficulty level of such games. They are so huge - in terms of items, locations, NPC, quests and everything else - that you really can't even imagine it without a couple of weeks of playing with them; it then takes years to complete the game for yourself, and if that's not enough there are challenge games (not using certain items, never eating, finishing game naked, stuff like that).

The essence of roguelikes is that of Diablo and other similar games: you're walking around killing things, picking up items and advancing levels. In most roguelikes, however, your freedom of movement, details of the environment, number of choices to make are on a whole another level. Why have 3 (or 5) PC classes, when you can have 20 or 22, each really unique in strengths and weaknesses, utilizing different tactics and strategies? And PC race matters too, so the number of available combinations is ~(12 times 22) before you even started playing!

One in-game example of how detailed and complex roguelikes tend to be: some levels are flooded and you can't get past water without knowing how to swim... Wait, no. You of course can go into the water, no matter the swimming skill. Without swimming skill - and with swimming not trained very high - you'll lose some HP due to drowning. You can reduce the chance of drowning by leaving your heavy equipment on the ground. On the other hand, going into the water in this shiny metal armor will make it rust. After a while. Note that there are metals that do not rust, these are safe. Also, the scrolls and magic books, if not wrapped in a waterproof blanket, become unusable after some time in the water. You also obviously don't want to try lightning based magic when standing in the water. But hey, you can just build a bridge! If you have a skill for it, a hatchet and some trees nearby. Or, and this is my favourite way, you can just freeze the water with (most of) ice based spells. But don't do this when being chased by a fire using enemy: your ice bridge can easily melt. Also, keep an eye on how much your equipment weights: bridges made of ice are not very solid. You could also get a pickaxe and try to dig your way around the water, but this takes time, so maybe a wand of digging would be better...

And that's the number of options available in only one of hundreds of different and interesting situation the game puts you in. It's an experience as close to classic table-top RPG as possible without a human as a dungeon master. It also makes you forever unimpressed with "interactive environments" and "complicated plots" of modern games.