Any "tag line" that Dwarf Fortress has seems like it should encompass the game as a whole, not simply a single part of it, like many of these suggestions do (tag lines about capturing mermaids, killer elephants, etc.) And i think that a good tag line should be immediately understandable by someone who has not played or even heard of the game.
I like "Blood for the blood god!" because it suggests the game has a lot of fighting/killing in it, which it does.
I also like "Go to Hell, make it yours," because it implies the ability to conquer stuff, which is definitely a big part of the game... especially once the army arc is added. However, it implies that "going" to hell is the GOAL of the game, when you in fact make your own goals.
Perhaps a tag line about megaprojects like inverted pyramids... "The Egyptians build pyramids... the dwarves can build them upside-down." Something like that, which demonstrates the value and fun of setting your own goals.
The classic "Strike the earth!" is also great. It's poetic. It tells you that the game is about mining, but the word "strike" also implies violence. And the game has plenty of that.
Perhaps a good tag line which mentions how addicting the game is?
Here's my contribution: "The dwarves dug too greedily and too deep." The line is stolen from Tolkien, whose fantasy elements have been a big inspiration in the game - a fantasy simulator. The word "greedily" shows that there are great riches to be had, and the words "too deep" have a forbidding and poetic ring to them... It's the classic dilemma of increasing risks vs. increasing rewards - a strong motif in the world of Dwarf Fortress.
But I still think that the ultimate and best slogan/tag line/hook/motto is still the one that has been given down to us from the great Toad... "Losing is fun." It explains so much about the game in just three words... that it's impossible to win, that you set your own goals instead, that not achieving these goals is STILL a source of fun, and most importantly that the whole game is simply ABOUT having fun, not winning - two things which many other games produced nowadays have got smushed together. It's not about the goal, it's about the journey. That's more than just a motto, it's a philosophy.
tl;dr: Losing is fun.