It's not as complex as Dwarf Fortress, but still has incredible depth in gameplay that still surprises me after probably hundreds of hours. For example, after thinking for the longest time that the wallets on zombies were useless flavor items, I found out just recently that you can actually deposit their cash at ATMs and use a credit card to buy gasoline at gas stations. Playing cataclysm is having small discoveries like this every play session. Also the crafting system is so incredibly complex, except for advanced weapons and junk food you can probably craft every single item in the game yourself with enough dedication, starting with sticks and stones and ending at complex machining and chemistry. While sometimes complex tasks can get tedious, there always is a way to do things smarter and more efficiently. Once you figure out how the interactions with the game world are implemented, you can probably emulate most real world actions you could think of in any given scenario, like throwing your empty gun at an enemy's head or using a clothing rack as a makeshift melee weapon.