More or less, if you go to the main page of Dwarf Fortress (
http://bay12games.com/dwarves/), the entire page is what has changed.
Almost immediately after you left, we started the Sponsorship drive, which added a TON of new animals to the game.
Along with that came the advent of farming dwarves, with improvements to milking, cheesemaking, the addition of shearing woolly beasts, the need to feed grazing animals by pasturing them, the addition of bees, wax making, and honey harvesting. Oh, and egg laying.
Then some time passed, a lot of bugs got squashed, some more popped up. Game should run faster now, but new stuff also makes it run slower in some cases.
The site finder got improved, so you can search for what kind of place you want a little easier now.
Clay, pottery, earthenware, and glazing was added. Hospital bugs were fixed...
And then we got another major release.
Huge changes to adventure mode, to create a rudimentary economy, town growth, sewer systems, bandits.
The addition of more "night creatures", including werewolves and vampires that can come to your fortress in disguise.
The ability for spellcasters to learn "the secrets of life and death", which give them the ability to raise the undead and attack fortresses.
Tons of other neat stuff, new creatures, rewrite to the way dragonfire works (interaction system), the ability to ingest poisons and syndromes now, and lots of bugfixes. Oh, and now "evil" biomes raise dead on their own, get some neat weather effects, and have crazy eyeball grass.
But this is all summarized on the front page at
http://bay12games.com/dwarves/, just scroll to the bottom and read up. Stuff with bullet points is fixes/additions.