A 40d question:
What happens to your fort when you abandon it? I've gotten to the point where I'm almost falling asleep between sieges and just praying that they'll put up a fight for once. I did have some fun capturing an elf master lasher who was leading the siege in a cage, stripping him down, and putting him on display in a fancy statue garden. Besides that, though, it's been "Charcoal stock okay, seed stock okay, miners are mining, caravan's leaving... repeat". I am really attached to my dwarves, though. I doubt I'll ever forget Edem, the one eyed axedwarf who takes out about half of every incoming siege, or Momuz, the only noble who actually seems to care about the other dwarves.
Does abandoning your fort instantly bring it to a broken down, decayed state full of monsters to fight on a reclaim, or does it allow the dwarves to live on with their lives, doing things other dwarven cities would do? Alternatively, do sieges ever become powerful enough to take out an 11 champion military? The dragon that came by earlier never managed to even scratch one of my dwarves.
EDIT: To rephrase my intentions a little better: I want my fortress to die a natural death, whether through invasion, famine, or HFS. I want to be able to look over the history of my dwarves and see how they died and when. I'm just a little worried that once I abandon, they'll all die off or start behaving completely differently.