I mainly love DF for the depth of choice and complexity. In most games, you build X workshop over the X slot.... then you build the trap in the trap slot/choose from the list of predetermined traps. While DF still does have traps you build (stone, weapon, cage), we have the freedom to design our own. I'm talking about drowning chambers, pit-fall traps, garbage drops, minecraft loops, MAGMA!, etc. We can even go crazy with elaborate designs that include winding bridges over deep (zombie filled) chasms while under fire by marksdorfs protected by the empty 20z-level drop and fortifactions from the sides and facing a ballistae battery from the front. If anything survives this, there is a war bear/cave croc/jabberer "pasture" waiting for them before they fight your drunken steel-clad axelords... but only a few enemies at a time because you use "airlock" bridges to break up your enemies.
More than this, you can weaponize damn near anything. Not only are there vampire wells to make an entire fortress of super-soliders, but using GCS to make webbed traps to stop all but a handful of beasties. Or you could take that fire ball flinging titan and corral him into a fortification lined room overlooking your fort. When enemies appear, you lower the bridge that was blocking line of sight and let him have at it! You could put other FB's at the bottom of pits and feed him hippies! You can tame dragons! You can use a captured necromancer to make and endless wave of fodder to wear down sieges. You can force a syndrome-spewing FB to coat your masterwork weapons in heart stopping poison to use against your foes. You can build doors holding in magma to trick trolls into incinerating themselves... ah the possibilities are endless!
There is also the economy side of things. There are so many different overlapping industries. We can thus adjust our forts to suit the environment we are in. We can survive in lush jungles, scorching deserts, barren wastelands, or frozen glaciers. No metals? No problem! Use trap designs like I listed above until you can trade for metals/goblin christmass arrives. Don't care to farm? Gather herbs/pick trees and fish in the rivers. Press them olives into oil to make soap and then cook the pumice with honey and seeds to make full meals. Or just spam plump helmets and tell your dorfs to stop bitching when they want "diversity." Melt sand into glass and use them to make crafts that you decorate with rock gems :p Do nothing but fish and trade for booze-plants. Hunt and make bone crossbows/leather armor. Turtle up and dig down to the CANDY to truly mince gobbies. Keep your dorfs happy with mist generators in your pimped out dining room.
There is simply too much depth to the game and freedom for creation. Not only can we just adapt on the fly what industries, defense strategies, and food production to use but we can pick our own challenges to see an entirely different game (settle in area without metal, no using "traps," embark with only a pick, never use metal, no farms, etc). You can play the game however you want and still scrape by. There are no set paths or hand holding either, we make the game what we want it to be- THAT is why I love DF.