[Update #2]: Dwarf Fortress' new and improved version has now landed onto Steam, and I'm happy to say that it has done so alongside overwhelmingly positive reviews!.
[Update]: Dwarf Fortress devs have revealed the upcoming mouse-driven UI improvements!
Despite looking like a rejected game concept from the 1980's, Dwarf Fortress is incredibly complex as it runs simulations for nearly every event that happens within the game world, even the absurd stuff such as drunk Dwarves staggering about. Naturally, all of this is quite taxing on the performance and its not unusual for the frame rate in longer campaigns to dip down considerably, hence the constantly ongoing efforts to modernize the Dwarf Fortress client.
While the recent update to 64-bits most likely won't result in much of a speed boost just yet, it will allow you to create bigger and older worlds, though at the cost of even longer loading times, but such are the sacrifices you make when you want to create the greatest fortress in the world! Since the patch notes for Update 0.43.05 are quite brief, here's the whole thing:
New stuff
(*) 64-bit support, pulling the game from the distant past into the previous decade
Major bug fixes
(*) Fixed problem with artistic skill assignment causing world histories to diverge
(*) Fixed problem with worldgen trade causing world histories to diverge
Other bug fixes/tweaks
(*) Stopped babies/children from competing in w.g. events
(*) Made gorlaks able to open doors, stopped desizing of their heads
(*) Fixed some mem leaks
If you're interested in a "behind the scenes" look at the creation of the 64-bit version, allow me to direct you to this forum thread. Throughout its 28 pages you will be able to find numerous back-and-forths between Toady One (the developer) and various community members on what sort of features to implement, and how to actually do so. Its all highly technical stuff, mind you, but its still quite fascinating to see the 64-bit client slowly taking shape.
Finally, if you're interested in playing Dwarf Fortress and learning why its community shivers in fear whenever anyone utters "catsplosion", you can do so by downloading the free client over here. Also, make sure to either look up some guides on Youtube or at the very least play with the wiki open, because to do anything else would be pure madness given Dwarf Fortress' nearly insurmountable complexity.