I love builder games, and games where the player can change parameters that affect the physical properties of the generated world.
In DF, trees grow, crops produce seeds, and thus the renewable resources renew themselves. This is important to me. I dislike games that run out of resources when they think the game should be over for the player.
DF, at least in this version, has no Game Over condition. I can play as long as I want to play. I am done when I decide that I am done.
I got hooked on DF the first time because it simulates engineering in ways I had never seen in a game before. For instance, there is no other builder game, that I know of, that allows me to pump fluids where I will.
A year later, after having spent part of a year playing a multi-player wargame online with my husband, I got hooked on DF a second time because it generates a detailed history behind every world. I love reading legends generated without pruning the ancestry tree.
To all of this, add the ability to mod the game raws and DF is a winner for me. Tarn Adams is working to improve all of these features, so unless he makes a really bad error from my point of view, I am here to stay as a fan of DF.
Other builder games which have stood the test of time with me, but fail in one way or another as compared to DF are:
Caeser 3 (C4 is prettier but less adaptable to sandbox play)
Pharaoh (suffers from "ghost" population which eventually causes the city to fail)
Children of the Nile (this one is probably my favorite on this list but the constant requirement to keep "prestige" from falling limits play time)
Civ 4 (and earlier versions)
Alpha Centauri
All of the games on the list are prettier than DF, but none offer much opportunity to modify the game other than drawing custom maps or, in the case of Alpha Centauri, making custom races.
MOO2 was also a favorite of mine but I have not played it in years. Startopia may make it back onto my hard drive this year.
Neverwinter Nights offers great modability both in graphics and in scripting, and I have built entire worlds with that game, but of course it is an RPG and not at all in the same category.