Personally, I am looking forward not only to the addition of the new and exciting content, such as the underground features and societies, but to the improvement of control over your dwarfs.
I think for many people who play dwarf fortress, controlling the dwarfs in your fortress is a key component of the game. The game starts with 7 dwarfs that have unique personalities. Your fortress grows upon these few dwarfs; if they do their jobs correctly, then you survive another winter. For the first few games, the new player is dependent on a very small selection of dwarfs as he learns the game.
But then some immigrants come in with a strange assortment of professions... then more immigrants, and more... till your fortress houses 50 odd dwarfs all running amok (not to be confused with the blood god Armok). The rations are all eaten, there is no more booze left, people no longer have rooms and the fps is now at a steady 20fps.
It is now an issue controlling all those dwarfs. Trying to create order in your fortress becomes a difficult task. Trying to equip your military, to use the correct weapons and armor is a headache. Setting up the different tasks each dwarf is to handle is cumbersome, the best options are to use a different program or to just let the new dwarfs be.
For me, the game losses its fun as the fortress develops because I no longer have control over the fortress. (Note: I haven't utilize what tools already included in DF to help manage the fortress) Where once we had individual dwarfs that we watched and cared about, we now have a swarm of random, uncontrolled dwarfs running around everywhere.